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+"""
+567. Permutation in String
+Difficulty: Medium
+https://leetcode.com/problems/permutation-in-string/
+
+──────────────────────────────────────────────────
+
+Given two strings s1 and s2, return true if s2 contains a permutation
+of s1, or false otherwise.
+
+In other words, return true if one of s1's permutations is the
+substring of s2.
+
+
+
+Example 1:
+
+Input: s1 = "ab", s2 = "eidbaooo"
+Output: true
+Explanation: s2 contains one permutation of s1 ("ba").
+
+Example 2:
+
+Input: s1 = "ab", s2 = "eidboaoo"
+Output: false
+
+
+
+Constraints:
+
+        • 1 <= s1.length, s2.length <= 10^4
+
+        • s1 and s2 consist of lowercase English letters.
+"""
+
+from collections import Counter
+
+
+class Solution:
+    def checkInclusion(self, s1: str, s2: str) -> bool:
+        k = len(s1)
+        if k > len(s2):
+            return False
+
+        need = Counter(s1)
+        window = Counter()
+
+        for right, c in enumerate(s2):
+            window[c] += 1
+            if right >= k:
+                left_char = s2[right - k]
+                window[left_char] -= 1
+                if window[left_char] == 0:
+                    del window[left_char]
+            if window == need:
+                return True
+
+        return False