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added Substring with Exactly K Distinct Characters

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`@@ -130,6 +130,7 @@ Languages used: Java and Python`
`130`	`130`	`- [Minimum Steps to Make Two Strings Anagram](String/MinStepsToMakeTwoStringsAnagram)`
`131`	`131`	`- [Rearrange Words in Sentence](String/RearrangeWordsInSentence)`
`132`	`132`	`- [String Matching in Array](String/StringMatchingInArray)`
	`133`	`+- [Substring with Exactly K Distinct Characters](String/SubstrWithExactKDistinctChars)`
`133`	`134`
`134`	`135`	`### [Tree](./Tree)`
`135`	`136`

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	`1`	`+# Substrings with exactly K distinct chars`
	`2`	`+`
	`3`	`+[Leetcode Link](https://leetcode.com/discuss/interview-question/370157)`
	`4`	`+`
	`5`	`+## Problem:`
	`6`	`+`
	`7`	`+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.`
	`8`	`+`
	`9`	`+## Example:`
	`10`	`+`
	`11`	+```
	`12`	`+Input: s = "pqpqs", k = 2`
	`13`	`+Output: 7`
	`14`	`+Explanation: ["pq", "pqp", "pqpq", "qp", "qpq", "pq", "qs"]`
	`15`	+```
	`16`	`+`
	`17`	+```
	`18`	`+Input: s = "aabab", k = 3`
	`19`	`+Output: 0`
	`20`	+```
	`21`	`+`
	`22`	`+## Note:`
	`23`	`+`
	`24`	`+- The input string consists of only lowercase English letters [a-z]`
	`25`	`+- 0 ≤ k ≤ 26`

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	`1`	`+def substrKDistinctChars(s, k):`
	`2`	`+ # result = list()`
	`3`	`+ result = 0`
	`4`	`+ for i in range(len(s)):`
	`5`	`+ distinctChars = set({s[i]})`
	`6`	`+ # subString = s[i]`
	`7`	`+ for j in range(i+1, len(s)):`
	`8`	`+ if len(distinctChars) < k or (len(distinctChars) == k and s[j] in distinctChars):`
	`9`	`+ distinctChars.add(s[j])`
	`10`	`+ # subString += s[j]`
	`11`	`+ else:`
	`12`	`+ break`
	`13`	`+ if len(distinctChars) == k:`
	`14`	`+ # result.append(subString)`
	`15`	`+ result += 1`
	`16`	`+ return result`
	`17`	`+`
	`18`	`+`
	`19`	`+# test case`
	`20`	`+s = "pqpqs"`
	`21`	`+k = 2`
	`22`	`+print(substrKDistinctChars(s, k))`

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