Given a string `s` and an int `k`, return an int representing the number of substrings (not unique) of `s` with exactly `k` distinct characters. If the given string doesn\'t have `k` distinct characters, return 0.\nhttps://leetcode.com/problems/subarrays-with-k-different-integers\n\n**Example 1:**\n```\nInput: s = "pqpqs", k = 2\nOutput: 7\nExplanation: ["pq", "pqp", "pqpq", "qp", "qpq", "pq", "qs"]\n```\n\n**Example 2:**\n```\nInput: s = "aabab", k = 3\nOutput: 0\n```\n\nConstraints:\n* The input string consists of only lowercase English letters `[a-z]`\n* 0 \u2264 `k` \u2264 26

Given a string s and an int k, return an int representing the number of substrings (not unique) of s with exactly k distinct characters. If the given string doe