Unique Substrings With Equal Digit Frequency — Python

# Time:  O(n^2)
# Space: O(n^2)

import collections


# rolling hash
class Solution(object):
    def equalDigitFrequency(self, s):
        """
        :type s: str
        :rtype: int
        """
        MOD = 10**9+7
        D = 27
        lookup = set()
        for i in xrange(len(s)):
            cnt = collections.Counter()
            h = max_cnt = 0
            for j in xrange(i, len(s)):
                d = ord(s[j])-ord('0')+1
                h = (h*D+d)%MOD
                cnt[d] += 1
                max_cnt = max(max_cnt, cnt[d])
                if len(cnt)*max_cnt == j-i+1:
                    lookup.add(h)
        return len(lookup)