go: add «2537. Count the Number of Good Subarrays»

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

go/count-the-number-of-good-subarrays.go

@@ -0,0 +1,24 @@
+package main
+
+func countGood(nums []int, k int) int64 {
+	counters := make(map[int]int)
+	count := int64(0)
+
+	same, right := 0, -1
+	for _, x := range nums {
+		for same < k && right+1 < len(nums) {
+			right++
+			same += counters[nums[right]]
+			counters[nums[right]]++
+		}
+
+		if same >= k {
+			count += int64(len(nums) - right)
+		}
+
+		counters[x]--
+		same -= counters[x]
+	}
+
+	return count
+}