Add solution #519

Author: JoshCrozier

README.md

@@ -415,6 +415,7 @@
 516|[Longest Palindromic Subsequence](./0516-longest-palindromic-subsequence.js)|Medium|
 517|[Super Washing Machines](./0517-super-washing-machines.js)|Hard|
 518|[Coin Change II](./0518-coin-change-ii.js)|Medium|
+519|[Random Flip Matrix](./0519-random-flip-matrix.js)|Medium|
 520|[Detect Capital](./0520-detect-capital.js)|Easy|
 521|[Longest Uncommon Subsequence I](./0521-longest-uncommon-subsequence-i.js)|Easy|
 530|[Minimum Absolute Difference in BST](./0530-minimum-absolute-difference-in-bst.js)|Easy|

solutions/0519-random-flip-matrix.js

@@ -0,0 +1,47 @@
+/**
+ * 519. Random Flip Matrix
+ * https://leetcode.com/problems/random-flip-matrix/
+ * Difficulty: Medium
+ *
+ * There is an m x n binary grid matrix with all the values set 0 initially. Design an algorithm
+ * to randomly pick an index (i, j) where matrix[i][j] == 0 and flips it to 1. All the indices
+ * (i, j) where matrix[i][j] == 0 should be equally likely to be returned.
+ *
+ * Optimize your algorithm to minimize the number of calls made to the built-in random function
+ * of your language and optimize the time and space complexity.
+ *
+ * Implement the Solution class:
+ * - Solution(int m, int n) Initializes the object with the size of the binary matrix m and n.
+ * - int[] flip() Returns a random index [i, j] of the matrix where matrix[i][j] == 0 and flips
+ *   it to 1.
+ * - void reset() Resets all the values of the matrix to be 0.
+ */
+
+/**
+ * @param {number} m
+ * @param {number} n
+ */
+var Solution = function(m, n) {
+  this.m = m;
+  this.n = n;
+  this.total = m * n;
+  this.flipped = new Map();
+};
+
+/**
+ * @return {number[]}
+ */
+Solution.prototype.flip = function() {
+  const index = Math.floor(Math.random() * this.total--);
+  const result = this.flipped.get(index) ?? index;
+  this.flipped.set(index, this.flipped.get(this.total) ?? this.total);
+  return [Math.floor(result / this.n), result % this.n];
+};
+
+/**
+ * @return {void}
+ */
+Solution.prototype.reset = function() {
+  this.total = this.m * this.n;
+  this.flipped.clear();
+};