added Substring with Exactly K Distinct Characters

Author: minhyeong-joe

README.md

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

String/SubstrWithExactKDistinctChars/README.md

@@ -0,0 +1,25 @@
+# Substrings with exactly K distinct chars
+
+[Leetcode Link](https://leetcode.com/discuss/interview-question/370157)
+
+## Problem:
+
+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.
+
+## Example:
+
+```
+Input: s = "pqpqs", k = 2
+Output: 7
+Explanation: ["pq", "pqp", "pqpq", "qp", "qpq", "pq", "qs"]
+```
+
+```
+Input: s = "aabab", k = 3
+Output: 0
+```
+
+## Note:
+
+- The input string consists of only lowercase English letters [a-z]
+- 0 ≤ k ≤ 26

String/SubstrWithExactKDistinctChars/solution.py

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+def substrKDistinctChars(s, k):
+    # result = list()
+    result = 0
+    for i in range(len(s)):
+        distinctChars = set({s[i]})
+        # subString = s[i]
+        for j in range(i+1, len(s)):
+            if len(distinctChars) < k or (len(distinctChars) == k and s[j] in distinctChars):
+                distinctChars.add(s[j])
+                # subString += s[j]
+            else:
+                break
+            if len(distinctChars) == k:
+                # result.append(subString)
+                result += 1
+    return result
+
+
+# test case
+s = "pqpqs"
+k = 2
+print(substrKDistinctChars(s, k))