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add 27 java, update cheatsheet
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‎README.md‎

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015 | [3 Sum](https://leetcode.com/problems/3sum/) | [Python](./leetcode_python/Array/3sum.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/ThreeSum.java)| _O(n^2)_ | _O(1)_ | Medium | Curated Top 75, good basic, check with `# 001 Two Sum`, `#018 4 sum`, `Two Pointers`, `good basic`,`amazon`,`fb`| OK******* (7)
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016 | [3 Sum Closest](https://leetcode.com/problems/3sum-closest/) | [Python](./leetcode_python/Array/3sum-closest.py) | _O(n^2)_ | _O(1)_ | Medium |`Two Pointers`, `basic`, `good trick`,`3 sum`,google, fb, apple, m$, GS, amazon| AGAIN**** (2)
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018| [4 Sum](https://leetcode.com/problems/4sum/) | [Python](./leetcode_python/Array/4sum.py) | _O(n^3)_ | _O(1)_ | Medium | `k sum`, `Two Pointers`, check `#016 3 sum`, `# 454 4 SUM II`,good trick, `fb` | AGAIN******** (4)
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026 | [Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)| [Python](./leetcode_python/Array/remove-duplicates-from-sorted-array.py), [Scala](./leetcode_scala/Array/removeDuplicates.scala), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/RemoveDuplicatesFromSortedArray.java) | _O(n)_ | _O(1)_| Easy | `Two Pointers`, `basic`, `good trick`,`M$` ,`fb` | AGAIN************** (11)
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027 | [Remove Element](https://leetcode.com/problems/remove-element/) | [Python](./leetcode_python/Array/remove-element.py), [Scala](./leetcode_scala/Array/removeElement.scala) | _O(n)_ | _O(1)_ | Easy |`basic`, `amazon`| OK* (2)
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026 | [Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)| [Python](./leetcode_python/Array/remove-duplicates-from-sorted-array.py), [Scala](./leetcode_scala/Array/removeDuplicates.scala), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/RemoveDuplicatesFromSortedArray.java) | _O(n)_ | _O(1)_| Easy | MUST, `Two Pointers`, `basic`, `good trick`,`M$` ,`fb` | AGAIN************** (11)
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027 | [Remove Element](https://leetcode.com/problems/remove-element/) | [Python](./leetcode_python/Array/remove-element.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/RemoveElement.java) | _O(n)_ | _O(1)_ | Easy |MUST, `2 pointers`, `amazon`| OK* (2)
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031 | [Next Permutation](https://leetcode.com/problems/next-permutation/)| [Python](./leetcode_python/Array/next-permutation.py), [Scala](./leetcode_scala/Array/nextPermutation.scala), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/NextPermutation.java)| _O(n)_ | _O(1)_ | Medium | good trick, check, 2 pointers, Uber, GS, `google`,`amazon`, `fb` | AGAIN****************** (8) (MUST)
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041 | [First Missing Positive](https://leetcode.com/problems/first-missing-positive/)| [Python](./leetcode_python/Array/first-missing-positive.py)| _O(n)_ | _O(1)_ | Hard | good trick, hash key , array, LC top 100 like, `amazon`, `fb`, google, apple, uber | AGAIN******** (2)
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048 | [Rotate Image](https://leetcode.com/problems/rotate-image/) | [Python](./leetcode_python/Array/rotate-image.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/Array/RotateImage.java) | _O(n^2)_ | _O(1)_| Medium |Curated Top 75, `basic`, `i,j->j,i` `transpose matrix`, garena, `amazon`, `apple`| AGAIN************* (5) (MUST)

‎data/progress.txt‎

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20241103: 26,27
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20241103:
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20241102: 26,27
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20241102: 802,1197,26
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20241027: 855,846
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20241026: 932

‎doc/cheatsheet/2_pointers.md‎

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// java
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// LC 26 (LC 83)
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// https://labuladong.online/algo/essential-technique/array-two-pointers-summary/#%E5%8E%9F%E5%9C%B0%E4%BF%AE%E6%94%B9
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/**
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* //--------------------------------
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* Example 1
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* //--------------------------------
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*
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* nums = [1,1,2]
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*
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* [1,1,2]
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* s f
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*
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* [1,2, 1] if nums[f] != nums[s], move s, then swap f, s
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* s s f
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*
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*
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* //--------------------------------
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* Example 2
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* //--------------------------------
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*
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* nums = [0,0,1,1,1,2,2,3,3,4]
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*
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* [0,0,1,1,1,2,2,3,3,4]
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* s f
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*
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* [0,1,0,1,1,2,2,3,3,4] if nums[f] != nums[s], move s, then swap f, s
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* s s f
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*
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* [0,1,0,1,1,2,2,3,3,4]
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* s f
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*
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* [0,1,0,1,1,2,2,3,3,4]
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* s f
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*
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* [0,1,2,1,1,0,2,3,3,4] if nums[f] != nums[s], move s, then swap f, s
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* s s f
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*
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* [0,1,2,1,1,0,2,3,3,4]
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* s f
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*
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* [0,1,2,3,1,0,2,1,3,4] if nums[f] != nums[s], move s, then swap f, s
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* s s f
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*
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* [0,1,2,3,1,0,2,1,3,4]
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* s f
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*
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* [0,1,2,3,4,0,2,1,3,1] if nums[f] != nums[s], move s, then swap f, s
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* s s f
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*
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*/
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class Solution {
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public int removeDuplicates(int[] nums) {
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if (nums.length == 0) {
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// java
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// LC 27
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// https://labuladong.online/algo/essential-technique/array-two-pointers-summary/#%E5%8E%9F%E5%9C%B0%E4%BF%AE%E6%94%B9
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/**
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* //--------------------
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* Example 1
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* //--------------------
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*
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* nums = [3,2,2,3], val = 3
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*
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* [3,2,2,3]
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* s
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* f
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*
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* [2,3,2,3] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [2,2,3,3] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [2,2,3,3]
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* s
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* f
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*
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*
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* //--------------------
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* Example 2
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* //--------------------
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*
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* nums = [0,1,2,2,3,0,4,2], val = 2
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*
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*
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* [0,1,2,2,3,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,2,2,3,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,2,2,3,0,4,2]
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* s
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* f
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*
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* [0,1,2,2,3,0,4,2]
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* s
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* f
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*
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* [0,1,3,2,2,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,2,2,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,4,2,2,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,4,2,2,2]
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* s
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* f
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*/
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class Solution {
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public int removeElement(int[] nums, int val) {
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int fast = 0, slow = 0;

‎leetcode_java/src/main/java/LeetCodeJava/Array/RemoveElement.java‎

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package LeetCodeJava.Array;
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// https://leetcode.com/problems/remove-element/description/
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/**
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* 27. Remove Element
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Solved
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Easy
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Topics
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Companies
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Hint
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Given an integer array nums and an integer val, remove all occurrences of val in nums in-place. The order of the elements may be changed. Then return the number of elements in nums which are not equal to val.
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Consider the number of elements in nums which are not equal to val be k, to get accepted, you need to do the following things:
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Change the array nums such that the first k elements of nums contain the elements which are not equal to val. The remaining elements of nums are not important as well as the size of nums.
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Return k.
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Custom Judge:
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The judge will test your solution with the following code:
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int[] nums = [...]; // Input array
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int val = ...; // Value to remove
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int[] expectedNums = [...]; // The expected answer with correct length.
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// It is sorted with no values equaling val.
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int k = removeElement(nums, val); // Calls your implementation
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assert k == expectedNums.length;
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sort(nums, 0, k); // Sort the first k elements of nums
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for (int i = 0; i < actualLength; i++) {
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assert nums[i] == expectedNums[i];
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}
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If all assertions pass, then your solution will be accepted.
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Example 1:
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Input: nums = [3,2,2,3], val = 3
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Output: 2, nums = [2,2,_,_]
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Explanation: Your function should return k = 2, with the first two elements of nums being 2.
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It does not matter what you leave beyond the returned k (hence they are underscores).
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Example 2:
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Input: nums = [0,1,2,2,3,0,4,2], val = 2
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Output: 5, nums = [0,1,4,0,3,_,_,_]
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Explanation: Your function should return k = 5, with the first five elements of nums containing 0, 0, 1, 3, and 4.
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Note that the five elements can be returned in any order.
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It does not matter what you leave beyond the returned k (hence they are underscores).
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Constraints:
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0 <= nums.length <= 100
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0 <= nums[i] <= 50
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0 <= val <= 100
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*
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*/
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class RemoveElement {
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// V0
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// IDEA : 2 POINTERS
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/**
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* //--------------------
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* Example 1
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* //--------------------
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*
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* nums = [3,2,2,3], val = 3
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*
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* [3,2,2,3]
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* s
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* f
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*
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* [2,3,2,3] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [2,2,3,3] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [2,2,3,3]
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* s
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* f
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*
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*
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* //--------------------
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* Example 2
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* //--------------------
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*
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* nums = [0,1,2,2,3,0,4,2], val = 2
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*
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*
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* [0,1,2,2,3,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,2,2,3,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,2,2,3,0,4,2]
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* s
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* f
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*
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* [0,1,2,2,3,0,4,2]
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* s
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* f
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*
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* [0,1,3,2,2,0,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,2,2,4,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,4,2,2,2] if nums[f] != val, swap, move s
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* s s
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* f
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*
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* [0,1,3,0,4,2,2,2]
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* s
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* f
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*/
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public int removeElement(int[] nums, int val) {
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int s = 0;
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for (int f = 0; f < nums.length; f++){
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if (nums[f] != val){
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nums[s] = nums[f];
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s += 1;
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}
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}
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return s;
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}
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// V1
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// IDEA : 2 POINTERS
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// https://leetcode.com/problems/remove-element/solutions/3670940/best-100-c-java-python-beginner-friendly/
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public int removeElement_1(int[] nums, int val) {
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int index = 0;
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for (int i = 0; i < nums.length; i++) {
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if (nums[i] != val) {
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nums[index] = nums[i];
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index++;
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}
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}
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return index;
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}
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// V2
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// IDEA : 2 POINTERS
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// https://leetcode.com/problems/remove-element/solutions/3102906/java-best-solution-o-n-time-complexity/
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public int removeElement_2(int[] nums, int val) {
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int i = 0;
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for (int j = 0; j < nums.length; j++) {
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if (nums[j] != val) {
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int temp = nums[i];
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nums[i] = nums[j];
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nums[j] = temp;
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i++;
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}
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}
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return i;
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}
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}

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