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‎README.md‎

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463| [Island Perimeter](https://leetcode.com/problems/island-perimeter/)|[Python ](./leetcode_python/Hash_table/island_perimeter.py) | _O(n)_ | _O(k)_ | Medium |`basic`,`hash table`, `google`, `fb`, amazon| AGAIN** (2)
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470| [Implement Rand10() Using Rand7()](https://leetcode.com/problems/implement-rand10-using-rand7/) | [Python ](./leetcode_python/Hash_table/implement-rand10-using-rand7.py) | _O(1)_ | _O(1)_ | Medium |`trick`, `google`| AGAIN** (3)
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473| [Matchsticks to Square](https://leetcode.com/problems/matchsticks-to-square/) | [Python ](./leetcode_python/Hash_table/matchsticks-to-square.py) | _O(n * s * 2^n)_ | _O(n * (2^n + s))_ | Medium |`complex`,`hard`| AGAIN (not start)
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523| [Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/) | [Python ](./leetcode_python/Hash_table/continuous-subarray-sum.py) | _O(n)_ | _O(k)_ | Medium |sub array sum, check `# 560 Subarray Sum Equals K`,`good trick`,`substring`, `hash table`,`AGAIN`,`M$`,`fb`, `apple`| AGAIN*************** (10) (MUST)
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523| [Continuous Subarray Sum](https://leetcode.com/problems/continuous-subarray-sum/) | [Python ](./leetcode_python/Hash_table/continuous-subarray-sum.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/HashTable/ContinuousSubarraySum.java) | _O(n)_ | _O(k)_ | Medium |sub array sum, check `# 560 Subarray Sum Equals K`,`good trick`,`substring`, `hash table`,`AGAIN`,`M$`,`fb`, `apple`| AGAIN*************** (10) (MUST)
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525| [Contiguous Array](https://leetcode.com/problems/contiguous-array/) | [Python](./leetcode_python/Hash_table/contiguous-array.py) | _O(n)_ | _O(n)_ | Medium | sub-array sum, `good basic`, `array`, `hashmap`, `cache`, `AGAIN`, `fb`, amazon| AGAIN*************** (11) (MUST)
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532| [K-diff Pairs in an Array](https://leetcode.com/problems/k-diff-pairs-in-an-array/) | [Python ](./leetcode_python/Hash_table/k-diff-pairs-in-an-array.py) | _O(n)_ | _O(n)_ | Medium |hash table, `basic`, `collections.Counter()`,`a-b =k -> a = k + b `, `amazon`| AGAIN********** (6)
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554| [Brick Wall](https://leetcode.com/problems/brick-wall/) | [Python ](./leetcode_python/Hash_table/brick-wall.py) | _O(n)_ | _O(m)_ | Medium |`trick`,`hash map`, `bloomberg`, `fb`, grab | AGAIN******** (5)

‎data/progress.txt‎

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20241208: 304,853,325,560(todo)
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20241214: 560(todo),523(todo)
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20241208: 304,853,325
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20241202: 370(todo),1109(todo)
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20241130: 34,767
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20241126: 722,380

‎data/to_review.txt‎

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2025-02-07 -> ['560(todo),523(todo)']
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2025-02-01 -> ['304,853,325']
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2025-01-26 -> ['370(todo),1109(todo)']
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2025-01-24 -> ['34,767']
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2025-01-20 -> ['722,380']
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2025-01-19 -> ['33,81']
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2025-01-17 -> ['253']
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2025-01-17 -> ['560(todo),523(todo)', '253']
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2025-01-16 -> ['776,31']
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2025-01-15 -> ['004(todo),34(todo),162(todo),275(todo)']
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2025-01-14 -> ['986(todo),1229(todo),1868(todo),80(todo),209(todo),283(todo),360(todo),713(todo),532(todo),611(todo)']
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2025-01-11 -> ['304,853,325', '394']
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2025-01-10 -> ['833,950']
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2025-01-05 -> ['370(todo),1109(todo)']
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2025-01-04 -> ['53,210,207']
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2025-01-04 -> ['560(todo),523(todo)', '53,210,207']
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2025-01-03 -> ['34,767', '444']
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2025-01-02 -> ['1188,130,855(again)']
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2024-12-30 -> ['722,380']
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2024-12-29 -> ['304,853,325', '33,81']
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2024-12-28 -> ['900']
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2024-12-27 -> ['253', '26,27', '802,1197,26']
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2024-12-27 -> ['560(todo),523(todo)', '253', '26,27', '802,1197,26']
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2024-12-26 -> ['776,31']
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2024-12-25 -> ['004(todo),34(todo),162(todo),275(todo)']
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2024-12-24 -> ['986(todo),1229(todo),1868(todo),80(todo),209(todo),283(todo),360(todo),713(todo),532(todo),611(todo)']
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2024-12-23 -> ['370(todo),1109(todo)']
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2024-12-22 -> ['560(todo),523(todo)']
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2024-12-21 -> ['304,853,325', '34,767', '394', '855,846']
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2024-12-20 -> ['833,950', '932']
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2024-12-19 -> ['560(todo),523(todo)']
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2024-12-18 -> ['951,792']
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2024-12-17 -> ['722,380']
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2024-12-16 -> ['304,853,325', '33,81']
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2024-12-15 -> ['370(todo),1109(todo)']
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2024-12-17 -> ['560(todo),523(todo)', '722,380']
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2024-12-16 -> ['560(todo),523(todo)', '304,853,325', '33,81']
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2024-12-15 -> ['560(todo),523(todo)', '370(todo),1109(todo)']
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2024-12-14 -> ['253', '53,210,207', '163,1048']
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2024-12-13 -> ['304,853,325', '34,767', '776,31', '444', '298,729']
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2024-12-12 -> ['004(todo),34(todo),162(todo),275(todo)', '1188,130,855(again)', '1146']

‎doc/pic/presum_mod.png‎

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‎leetcode_java/src/main/java/LeetCodeJava/HashTable/ContinuousSubarraySum.java‎

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package LeetCodeJava.HashTable;
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// https://leetcode.com/problems/continuous-subarray-sum/description/
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import java.util.HashMap;
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import java.util.Map;
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/**
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* 523. Continuous Subarray Sum
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* Solved
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* Medium
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* Topics
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* Companies
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* Given an integer array nums and an integer k, return true if nums has a good subarray or false otherwise.
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*
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* A good subarray is a subarray where:
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*
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* its length is at least two, and
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* the sum of the elements of the subarray is a multiple of k.
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* Note that:
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*
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* A subarray is a contiguous part of the array.
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* An integer x is a multiple of k if there exists an integer n such that x = n * k. 0 is always a multiple of k.
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*
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*
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* Example 1:
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*
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* Input: nums = [23,2,4,6,7], k = 6
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* Output: true
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* Explanation: [2, 4] is a continuous subarray of size 2 whose elements sum up to 6.
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* Example 2:
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*
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* Input: nums = [23,2,6,4,7], k = 6
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* Output: true
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* Explanation: [23, 2, 6, 4, 7] is an continuous subarray of size 5 whose elements sum up to 42.
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* 42 is a multiple of 6 because 42 = 7 * 6 and 7 is an integer.
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* Example 3:
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*
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* Input: nums = [23,2,6,4,7], k = 13
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* Output: false
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*
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*
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* Constraints:
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*
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* 1 <= nums.length <= 105
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* 0 <= nums[i] <= 109
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* 0 <= sum(nums[i]) <= 231 - 1
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* 1 <= k <= 231 - 1
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* Seen this question in a real interview before?
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* 1/5
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* Yes
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* No
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* Accepted
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* 625K
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* Submissions
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* 2M
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* Acceptance Rate
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* 30.5%
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*/
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public class ContinuousSubarraySum {
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// V0
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// IDEA : HASH MAP (fixed by gpt)
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public boolean checkSubarraySum(int[] nums, int k) {
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if (nums.length < 2) {
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return false;
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}
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// Map to store the remainder and its index
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Map<Integer, Integer> map = new HashMap<>();
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// Initialize with remainder 0 at index -1 to handle edge cases
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map.put(0, -1);
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int presum = 0;
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for (int i = 0; i < nums.length; i++) {
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presum += nums[i];
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// Calculate remainder
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int remainder = (k != 0) ? presum % k : presum;
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// If the remainder already exists in the map
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if (map.containsKey(remainder)) {
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// Check if the subarray length is at least 2
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if (i - map.get(remainder) > 1) {
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return true;
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}
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} else {
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// Store the first occurrence of this remainder
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map.put(remainder, i);
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}
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}
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return false;
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}
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// V1
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// IDEA : HASHMAP
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// https://leetcode.com/problems/continuous-subarray-sum/editorial/
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public boolean checkSubarraySum_1(int[] nums, int k) {
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int prefixMod = 0;
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HashMap<Integer, Integer> modSeen = new HashMap<>();
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modSeen.put(0, -1);
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for (int i = 0; i < nums.length; i++) {
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/**
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* NOTE !!! we get `mod of prefixSum`, instead of get prefixSum
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*/
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prefixMod = (prefixMod + nums[i]) % k;
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if (modSeen.containsKey(prefixMod)) {
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// ensures that the size of subarray is at least 2
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if (i - modSeen.get(prefixMod) > 1) {
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return true;
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}
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} else {
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// mark the value of prefixMod with the current index.
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modSeen.put(prefixMod, i);
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}
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}
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return false;
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}
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// V2
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// IDEA : HASHMAP
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// https://leetcode.com/problems/continuous-subarray-sum/solutions/1405425/a-java-implementation-of-prefix-sum-that-si0m/
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public boolean checkSubarraySum_2(int[] nums, int k) {
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// maintain a hash map to store <sum % k, index>
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Map<Integer, Integer> map = new HashMap<>();
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int sum = 0;
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for (int i = 0; i < nums.length; i++) {
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sum += nums[i];
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sum %= k;
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// case 1
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if (sum == 0 && i > 0) {
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return true;
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}
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// case 2
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if (map.containsKey(sum) && i - map.get(sum) > 1) {
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return true;
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}
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if (!map.containsKey(sum)) {
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map.put(sum, i);
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}
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}
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return false;
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}
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}

‎leetcode_java/src/main/java/dev/workspace5.java‎

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return maxSize;
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}
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// LC 523
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// https://leetcode.com/problems/continuous-subarray-sum/
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// 9.24 am - 9.40 am
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/**
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*
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* Given an integer array nums and an integer k,
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* return true if nums has a good subarray or false otherwise.
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*
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* A good subarray is a subarray where:
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*
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* its length is at least two, and
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* the sum of the elements of the subarray is a multiple of k.
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*
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*/
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public boolean checkSubarraySum(int[] nums, int k) {
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if (nums.length < 2){
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return false;
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}
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// hash map
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// record presum and idx
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// map(presum: idx)
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Map<Integer, Integer> map = new HashMap<>();
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int presum = 0;
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for (int i = 0; i < nums.length; i++){
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int cur = nums[i];
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// if there is any element equals 0, return true directly
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// if (cur == 0){
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// return true;
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// }
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presum += cur;
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map.putIfAbsent(presum, i);
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/**
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* sum(i,j) = presum(j+1) - presum(i)
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*
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* k = presum(j+1) - presum(i)
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* -> presum(i) = presum(j+1) - k
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*/
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if (map.containsKey(presum - k) || presum % k == 0){
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return true;
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}
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}
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return false;
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}
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44064452

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}

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