Permutation in String

固定长度的双指针题，核心思路还是RabinKarp型的移动字符串对比

class Solution(object):
    def checkInclusion(self, s1, s2):
        """
        :type s1: str
        :type s2: str
        :rtype: bool
        """
        m, n = len(s1), len(s2)
        dictM = collections.Counter(s1)
        dictN = collections.Counter(s2[:m])
        for i in xrange(n - m + 1):
            if dictM == dictN:
                return True
            if i < n - m:
                dictN[s2[i]] -= 1
                if not dictN[s2[i]]:
                    dictN.pop(s2[i])
                dictN[s2[i + m]] += 1

        return False