Add solution #493

Author: JoshCrozier

README.md

@@ -394,6 +394,7 @@
 488|[Zuma Game](./0488-zuma-game.js)|Hard|
 491|[Non-decreasing Subsequences](./0491-non-decreasing-subsequences.js)|Medium|
 492|[Construct the Rectangle](./0492-construct-the-rectangle.js)|Easy|
+493|[Reverse Pairs](./0493-reverse-pairs.js)|Hard|
 496|[Next Greater Element I](./0496-next-greater-element-i.js)|Easy|
 500|[Keyboard Row](./0500-keyboard-row.js)|Easy|
 501|[Find Mode in Binary Search Tree](./0501-find-mode-in-binary-search-tree.js)|Easy|

solutions/0493-reverse-pairs.js

@@ -0,0 +1,59 @@
+/**
+ * 493. Reverse Pairs
+ * https://leetcode.com/problems/reverse-pairs/
+ * Difficulty: Hard
+ *
+ * Given an integer array nums, return the number of reverse pairs in the array.
+ *
+ * A reverse pair is a pair (i, j) where:
+ * - 0 <= i < j < nums.length and
+ * - nums[i] > 2 * nums[j].
+ */
+
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var reversePairs = function(nums) {
+  let count = 0;
+  mergeSort(nums, 0, nums.length - 1);
+  return count;
+
+  function mergeSort(input, left, right) {
+    if (left >= right) return;
+    const middle = Math.floor((left + right) / 2);
+    mergeSort(input, left, middle);
+    mergeSort(input, middle + 1, right);
+    merge(input, left, middle, right);
+  }
+
+  function merge(input, left, middle, right) {
+    let j = middle + 1;
+    for (let i = left; i <= middle; i++) {
+      while (j <= right && input[i] > 2 * input[j]) {
+        j++;
+      }
+      count += j - (middle + 1);
+    }
+
+    const order = [];
+    let i = left;
+    j = middle + 1;
+    while (i <= middle && j <= right) {
+      if (input[i] <= input[j]) {
+        order.push(input[i++]);
+      } else {
+        order.push(input[j++]);
+      }
+    }
+    while (i <= middle) {
+      order.push(input[i++]);
+    }
+    while (j <= right) {
+      order.push(input[j++]);
+    }
+    for (let k = 0; k < order.length; k++) {
+      input[left + k] = order[k];
+    }
+  }
+};