add 279 java

Author: yennanliu

README.md

@@ -1049,7 +1049,7 @@
 221| [Maximal Square](https://leetcode.com/problems/maximal-square/)|[Python](./leetcode_python/Dynamic_Programming/maximal-square.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/DynamicProgramming/MaximalSquare.java) | _O(n^2)_   | _O(n)_ | Medium | EPI, dp, `amazon`,`fb`, google| AGAIN** (3) (not start)
 256| [Paint House](https://leetcode.com/problems/paint-house/) | [Python](./leetcode_python/Dynamic_Programming/paint-house.py) | _O(n)_| _O(1)_| Medium |🔒| AGAIN (not start) 
 276| [Paint Fence](https://leetcode.com/problems/paint-fence/) | [Python](./leetcode_python/Dynamic_Programming/paint-fence.py) | _O(n)_| _O(1)_| Easy |🔒| AGAIN 
-279| [Perfect Squares](https://leetcode.com/problems/perfect-squares/)|  [Python](./leetcode_python/Dynamic_Programming/perfect-squares.py) | _O(n * sqrt(n))_  | _O(n)_ | Medium | dp, bfs, trick, Hash, `google` | AGAIN* 
+279| [Perfect Squares](https://leetcode.com/problems/perfect-squares/)|  [Python](./leetcode_python/Dynamic_Programming/perfect-squares.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/DynamicProgramming/PerfectSquares.java) | _O(n * sqrt(n))_  | _O(n)_ | Medium | dp, bfs, trick, Hash, `google` | AGAIN* 
 303| [Range Sum Query - Immutable](https://leetcode.com/problems/range-sum-query-immutable/)| [Python](./leetcode_python/Dynamic_Programming/range-sum-query-immutable.py) | ctor: _O(n)_, lookup: _O(1)_  | _O(n)_          | Easy   || OK 
 304| [Range Sum Query 2D - Immutable](https://leetcode.com/problems/range-sum-query-2d-immutable/)| [Python](./leetcode_python/Dynamic_Programming/range-sum-query-2d-immutable.py), [Java](./leetcode_java/src/main/java/LeetCodeJava/DynamicProgramming/RangeSumQuery2DImmutable.java)  | ctor: _O(m * n)_, lookup: _O(1)_ | _O(m * n)_ | Medium |`dp`,`trick`,`fb`| AGAIN******* (5)
 309| [Best Time to Buy and Sell Stock with Cooldown](https://leetcode.com/problems/best-time-to-buy-and-sell-stock-with-cooldown/) | [Python](./leetcode_python/Dynamic_Programming/best-time-to-buy-and-sell-stock-with-cooldown.py) | _O(n)_ | _O(1)_ | Medium || AGAIN (not start)

data/progress.md

@@ -1,5 +1,9 @@
 # Progress
 
+# 2025-01-09
+- https://github.com/yennanliu/CS_basics/blob/master/doc/Leetcode_company_frequency-master/Google%206months-%20LeetCode.pdf
+- TODO: go through `neetcode`, note/summary knowledges (by topic) at cheatsheet
+
 # 2025-01-08
 - https://github.com/yennanliu/CS_basics/blob/master/doc/Leetcode_company_frequency-master/Google%206months-%20LeetCode.pdf
 - TODO: go through `neetcode`, note/summary knowledges (by topic) at cheatsheet

data/progress.txt

@@ -1,3 +1,4 @@
+20250109: 279,904
 20250108: 852,692
 20250107: 247
 20250105: 399,1091

leetcode_java/src/main/java/LeetCodeJava/DynamicProgramming/PerfectSquares.java

@@ -0,0 +1,124 @@
+package LeetCodeJava.DynamicProgramming;
+
+// https://leetcode.com/problems/perfect-squares/description/
+
+import java.util.Arrays;
+
+/**
+ * 279. Perfect Squares
+ * Solved
+ * Medium
+ * Topics
+ * Companies
+ * Given an integer n, return the least number of perfect square numbers that sum to n.
+ *
+ * A perfect square is an integer that is the square of an integer; in other words, it is the product of some integer with itself. For example, 1, 4, 9, and 16 are perfect squares while 3 and 11 are not.
+ *
+ *
+ *
+ * Example 1:
+ *
+ * Input: n = 12
+ * Output: 3
+ * Explanation: 12 = 4 + 4 + 4.
+ * Example 2:
+ *
+ * Input: n = 13
+ * Output: 2
+ * Explanation: 13 = 4 + 9.
+ *
+ *
+ * Constraints:
+ *
+ * 1 <= n <= 104
+ *
+ */
+public class PerfectSquares {
+
+    // V0
+//    public int numSquares(int n) {
+//
+//    }
+
+    // V1
+    // IDEA: DP
+    // https://leetcode.com/problems/perfect-squares/solutions/4694883/beats-99-users-cjavapythonjavascript-exp-37yg/
+    public int numSquares_1(int n) {
+        int[] dp = new int[n + 1];
+        Arrays.fill(dp, Integer.MAX_VALUE);
+        dp[0] = 0;
+        for (int i = 1; i <= n; ++i) {
+            int min_val = Integer.MAX_VALUE;
+            for (int j = 1; j * j <= i; ++j) {
+                min_val = Math.min(min_val, dp[i - j * j] + 1);
+            }
+            dp[i] = min_val;
+        }
+        return dp[n];
+    }
+
+    // V2
+    // https://leetcode.com/problems/perfect-squares/solutions/1520447/dp-easy-to-understand-js-java-python-c-b-sk59/
+    // IDEA: DP
+    public int numSquares_2(int n) {
+        int[] dp = new int[n + 1];
+        Arrays.fill(dp, Integer.MAX_VALUE);
+        dp[0] = 0;
+        int count = 1;
+        while (count * count <= n) {
+            int sq = count * count;
+            for (int i = sq; i <= n; i++) {
+                dp[i] = Math.min(dp[i - sq] + 1, dp[i]);
+            }
+            count++;
+        }
+        return dp[n];
+    }
+
+    // V3-1
+    // https://leetcode.com/problems/perfect-squares/solutions/2837992/java-recursion-memoization-dp-3-square-t-72qb/
+    // IDEA: Top Down DP (Recursion + Memoization)
+    public int numSquares_3_1(int n) {
+        int[] memo = new int[n + 1];
+        return helper(n, memo);
+    }
+
+    public int helper(int n, int[] memo) {
+        if (n < 4)
+            return n;
+
+        if (memo[n] != 0)
+            return memo[n];
+
+        int ans = n;
+
+        for (int i = 1; i * i <= n; i++) {
+            int square = i * i;
+            ans = Math.min(ans, 1 + helper(n - square, memo));
+        }
+
+        return memo[n] = ans;
+    }
+
+    // V3-2
+    // https://leetcode.com/problems/perfect-squares/solutions/2837992/java-recursion-memoization-dp-3-square-t-72qb/
+    // IDEA: Top Down DP (Recursion + Memoization)
+    // Time complexity: O(N * sqrt(N))
+    // Space complexity: O(N)
+    public int numSquares_3_2(int n) {
+        int[] dp = new int[n + 1];
+        dp[0] = 0;
+
+        for (int i = 1; i <= n; i++) {
+            dp[i] = i;
+
+            for (int j = 1; j * j <= i; j++) {
+                int square = j * j;
+                dp[i] = Math.min(dp[i], 1 + dp[i - square]);
+            }
+        }
+
+        return dp[n];
+    }
+
+}