Add solution #2563

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,360 LeetCode solutions in JavaScript
+# 1,361 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1287,6 +1287,7 @@
 2551|[Put Marbles in Bags](./solutions/2551-put-marbles-in-bags.js)|Hard|
 2559|[Count Vowel Strings in Ranges](./solutions/2559-count-vowel-strings-in-ranges.js)|Medium|
 2560|[House Robber IV](./solutions/2560-house-robber-iv.js)|Medium|
+2563|[Count the Number of Fair Pairs](./solutions/2563-count-the-number-of-fair-pairs.js)|Medium|
 2570|[Merge Two 2D Arrays by Summing Values](./solutions/2570-merge-two-2d-arrays-by-summing-values.js)|Easy|
 2579|[Count Total Number of Colored Cells](./solutions/2579-count-total-number-of-colored-cells.js)|Medium|
 2594|[Minimum Time to Repair Cars](./solutions/2594-minimum-time-to-repair-cars.js)|Medium|

solutions/2563-count-the-number-of-fair-pairs.js

@@ -0,0 +1,56 @@
+/**
+ * 2563. Count the Number of Fair Pairs
+ * https://leetcode.com/problems/count-the-number-of-fair-pairs/
+ * Difficulty: Medium
+ *
+ * Given a 0-indexed integer array nums of size n and two integers lower and upper, return the
+ * number of fair pairs.
+ *
+ * A pair (i, j) is fair if:
+ * - 0 <= i < j < n, and
+ * - lower <= nums[i] + nums[j] <= upper
+ */
+
+/**
+ * @param {number[]} nums
+ * @param {number} lower
+ * @param {number} upper
+ * @return {number}
+ */
+var countFairPairs = function(nums, lower, upper) {
+  let result = 0;
+
+  nums.sort((a, b) => a - b);
+
+  for (let i = 0; i < nums.length - 1; i++) {
+    const minTarget = lower - nums[i];
+    const maxTarget = upper - nums[i];
+
+    const first = findFirstValidIndex(i + 1, minTarget);
+    const last = findFirstValidIndex(i + 1, maxTarget + 1) - 1;
+
+    if (first <= last) {
+      result += last - first + 1;
+    }
+  }
+
+  return result;
+
+  function findFirstValidIndex(start, target) {
+    let left = start;
+    let right = nums.length - 1;
+    let index = nums.length;
+
+    while (left <= right) {
+      const mid = Math.floor((left + right) / 2);
+      if (nums[mid] >= target) {
+        index = mid;
+        right = mid - 1;
+      } else {
+        left = mid + 1;
+      }
+    }
+
+    return index;
+  }
+};