"""
567. Permutation in String
Difficulty: Medium
https://leetcode.com/problems/permutation-in-string/

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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