From cd0ad4d7d5e4a5f2c4df430994c1c47a945d61a1 Mon Sep 17 00:00:00 2001
From: Matej Focko <me@mfocko.xyz>
Date: Wed, 13 Nov 2024 17:19:49 +0100
Subject: [PATCH] =?UTF-8?q?kt:=20add=20=C2=AB2563.=20Count=20the=20Number?=
 =?UTF-8?q?=20of=20Fair=20Pairs=C2=BB?=
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URL:	https://leetcode.com/problems/count-the-number-of-fair-pairs/
Signed-off-by: Matej Focko <me@mfocko.xyz>
---
 kt/count-the-number-of-fair-pairs.kt | 34 ++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)
 create mode 100644 kt/count-the-number-of-fair-pairs.kt

diff --git a/kt/count-the-number-of-fair-pairs.kt b/kt/count-the-number-of-fair-pairs.kt
new file mode 100644
index 0000000..f4d5920
--- /dev/null
+++ b/kt/count-the-number-of-fair-pairs.kt
@@ -0,0 +1,34 @@
+class Solution {
+    private fun countFairPairsBelow(
+        nums: List<Int>,
+        upper: Int,
+    ): Long {
+        var pairs = 0L
+
+        var (left, right) = 0 to nums.size - 1
+        while (left < right) {
+            val sum = nums[left] + nums[right]
+
+            if (sum < upper) {
+                pairs += right - left
+                left += 1
+            } else {
+                right -= 1
+            }
+        }
+
+        return pairs
+    }
+
+    fun countFairPairs(
+        nums: IntArray,
+        lower: Int,
+        upper: Int,
+    ): Long {
+        val sortedNums = nums.sorted()
+        return (
+            countFairPairsBelow(sortedNums, upper + 1) -
+                countFairPairsBelow(sortedNums, lower)
+        )
+    }
+}