kt: add «2563. Count the Number of Fair Pairs»

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

@@ -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)
+        )
+    }
+}