Create 2563-count-the-number-of-fair-pairs.js

Author: everthis

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

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+/**
+ * @param {number[]} nums
+ * @param {number} lower
+ * @param {number} upper
+ * @return {number}
+ */
+const countFairPairs = function(nums, lower, upper) {
+  const n = nums.length
+  nums.sort((a, b) => a - b)
+  
+  let total = BigInt(n * (n - 1)) / 2n
+  return Number(total - BigInt(large(nums, upper) + low(nums, lower)))
+};
+
+
+function large(arr, target) {
+    let count = 0;
+    for (let lo = 0, hi = arr.length - 1; lo < hi; ) {
+        if (arr[lo] + arr[hi] > target) {
+            count += (hi - lo);
+            hi--;
+        } else  {
+            lo++;
+        }
+    }
+    return count;
+}
+
+function low(arr, target) {
+    let count = 0;
+    for (let lo = 0, hi = arr.length - 1; lo < hi; ) {
+        if (arr[lo] + arr[hi] < target) {
+            count += (hi - lo);
+            lo++;
+        } else  {
+            hi--;
+        }
+    }
+    return count;
+}