Permutation in String

def checkInclusion(s1, s2):
    if len(s1) > len(s2): return False
    
    s1Count, s2Count = [0] * 26, [0] * 26
    for i in range(len(s1)):
        s1Count[ord(s1[i]) - ord('a')] += 1
        s2Count[ord(s2[i]) - ord('a')] += 1
        
    matches = 0
    for i in range(26):
        matches += (1 if s1Count[i] == s2Count[i] else 0)
        
    l = 0
    for r in range(len(s1), len(s2)):
        if matches == 26: return True
        
        index = ord(s2[r]) - ord('a')
        s2Count[index] += 1
        if s1Count[index] == s2Count[index]:
            matches += 1
        elif s1Count[index] + 1 == s2Count[index]:
            matches -= 1
            
        index = ord(s2[l]) - ord('a')
        s2Count[index] -= 1
        if s1Count[index] == s2Count[index]:
            matches += 1
        elif s1Count[index] - 1 == s2Count[index]:
            matches -= 1
        l += 1
        
    return matches == 26