567. Leetcode solution in Python

#python

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Description:
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 <= 104

s1 and s2 consist of lowercase English letters.

Solution:

class Solution(object):
    def checkInclusion(self, s1, s2):
        """
        :type s1: str
        :type s2: str
        :rtype: bool
        """
        counts = collections.Counter(s1)
        l = len(s1)
        for i in xrange(len(s2)):
            if counts[s2[i]] > 0:
                l -= 1
            counts[s2[i]] -= 1
            if l == 0:
                return True
            start = i + 1 - len(s1)
            if start >= 0:  
                counts[s2[start]] += 1
                if counts[s2[start]] > 0:
                    l += 1
        return False