kt: add «2364. Count Number of Bad Pairs»

URL:	https://leetcode.com/problems/count-number-of-bad-pairs/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/count-number-of-bad-pairs.kt

@@ -0,0 +1,13 @@
+class Solution {
+    fun countBadPairs(nums: IntArray): Long =
+        mutableMapOf<Int, Int>().let { counters ->
+            nums.indices.sumOf { i ->
+                val diff = i - nums[i]
+                val goodPairs = counters.get(diff) ?: 0
+
+                counters[diff] = goodPairs + 1
+
+                (i - goodPairs).toLong()
+            }
+        }
+}