Add solution #398

Author: JoshCrozier

README.md

@@ -316,6 +316,7 @@
 395|[Longest Substring with At Least K Repeating Characters](./0395-longest-substring-with-at-least-k-repeating-characters.js)|Medium|
 396|[Rotate Function](./0396-rotate-function.js)|Medium|
 397|[Integer Replacement](./0397-integer-replacement.js)|Medium|
+398|[Random Pick Index](./0398-random-pick-index.js)|Medium|
 399|[Evaluate Division](./0399-evaluate-division.js)|Medium|
 404|[Sum of Left Leaves](./0404-sum-of-left-leaves.js)|Easy|
 405|[Convert a Number to Hexadecimal](./0405-convert-a-number-to-hexadecimal.js)|Easy|

solutions/0398-random-pick-index.js

@@ -0,0 +1,35 @@
+/**
+ * 398. Random Pick Index
+ * https://leetcode.com/problems/random-pick-index/
+ * Difficulty: Medium
+ *
+ * Given an integer array nums with possible duplicates, randomly output the index of a given
+ * target number. You can assume that the given target number must exist in the array.
+ *
+ * Implement the Solution class:
+ * - Solution(int[] nums) Initializes the object with the array nums.
+ * - int pick(int target) Picks a random index i from nums where nums[i] == target. If there
+ *   are multiple valid i's, then each index should have an equal probability of returning.
+ */
+
+/**
+ * @param {number[]} nums
+ */
+var Solution = function(nums) {
+  this.map = new Map();
+  for (let i = 0; i < nums.length; i++) {
+    if (!this.map.has(nums[i])) {
+      this.map.set(nums[i], []);
+    }
+    this.map.get(nums[i]).push(i);
+  }
+};
+
+/**
+ * @param {number} target
+ * @return {number}
+ */
+Solution.prototype.pick = function(target) {
+  const result = this.map.get(target);
+  return result[Math.floor(Math.random() * result.length)];
+};