add 519

Author: fishercoder1534

Diff for: paginated_contents/algorithms/1st_thousand/README.md

@@ -319,6 +319,7 @@
 | 522 | [Longest Uncommon Subsequence II](https://leetcode.com/problems/longest-uncommon-subsequence-ii/)                                                                                             | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_522.java)                                                                  |                                                                               | Medium                           |
 | 521 | [Longest Uncommon Subsequence I](https://leetcode.com/problems/longest-uncommon-subsequence-i/)                                                                                               | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_521.java)                                                                  |                                                                               | Easy                             |
 | 520 | [Detect Capital](https://leetcode.com/problems/detect-capital/)                                                                                                                               | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_520.java)                                                                  |                                                                               | Easy                             |
+| 519 | [Random Flip Matrix](https://leetcode.com/problems/random-flip-matrix/)                                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_519.java)                                                                  |                                                                               | Medium                           |HashTable, Math, Randomized, Reservoir Sampling 
 | 518 | [Coin Change 2](https://leetcode.com/problems/coin-change-2/)                                                                                                                                 | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_518.java)                                                                  |                                                                               | Medium                           | Array, DP                                                            
 | 517 | [Super Washing Machines](https://leetcode.com/problems/super-washing-machines/)                                                                                                               | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_517.java)                                                                  |                                                                               | Hard                             | DP                                                                   
 | 516 | [Longest Palindromic Subsequence](https://leetcode.com/problems/longest-palindromic-subsequence/)                                                                                             | [Solution](https://github.com/fishercoder1534/Leetcode/blob/master/src/main/java/com/fishercoder/solutions/firstthousand/_516.java)                                                                  |                                                                               | Medium                           | DP                                                                   

Diff for: src/main/java/com/fishercoder/solutions/firstthousand/_519.java

@@ -0,0 +1,36 @@
+package com.fishercoder.solutions.firstthousand;
+
+import java.util.HashSet;
+import java.util.Random;
+import java.util.Set;
+
+public class _519 {
+    public static class Solution {
+
+        private int m;
+        private int n;
+        private Set<Integer> flipped;
+        private Random random;
+
+        public Solution(int m, int n) {
+            this.m = m;
+            this.n = n;
+            this.random = new Random();
+            this.flipped = new HashSet<>();
+        }
+
+        public int[] flip() {
+            int i = random.nextInt(m);
+            int j = random.nextInt(n);
+            while (!flipped.add(i * n + j)) {
+                i = random.nextInt(m);
+                j = random.nextInt(n);
+            }
+            return new int[]{i, j};
+        }
+
+        public void reset() {
+            this.flipped = new HashSet<>();
+        }
+    }
+}

Diff for: src/test/java/com/fishercoder/firstthousand/_519Test.java

@@ -0,0 +1,21 @@
+package com.fishercoder.firstthousand;
+
+import com.fishercoder.common.utils.CommonUtils;
+import com.fishercoder.solutions.firstthousand._519;
+import org.junit.jupiter.api.Test;
+
+public class _519Test {
+
+    private static _519.Solution solution1;
+
+    @Test
+    public void test1() {
+        solution1 = new _519.Solution(3, 1);
+        CommonUtils.printArray(solution1.flip());
+        CommonUtils.printArray(solution1.flip());
+        CommonUtils.printArray(solution1.flip());
+        solution1.reset();
+        CommonUtils.printArray(solution1.flip());
+    }
+
+}