Count Substrings Divisible By Last Digit — Python

# Time:  O(d * n)
# Space: O(d)

# case works, math, freq table
class Solution(object):
    def countSubstrings(self, s):
        """
        :type s: str
        :rtype: int
        """
        result = 0
        # digit 1, 2, 5
        for i in xrange(len(s)):
            if s[i] in ('1', '2', '5'):
                result += i+1
        # digit 3, 6
        remain = 0
        cnt = [0]*3
        cnt[0] = 1
        for i in xrange(len(s)):
            remain = (remain+(ord(s[i])-ord('0')))%3
            if s[i] in ('3', '6'):
                result += cnt[remain]
            cnt[remain] += 1
        # digit 9
        remain = 0
        cnt = [0]*9
        cnt[0] = 1
        for i in xrange(len(s)):
            remain = (remain+(ord(s[i])-ord('0')))%9
            if s[i] == '9':
                result += cnt[remain]
            cnt[remain] += 1
        # digit 4
        for i in xrange(len(s)):
            if s[i] == '4':
                result += 1
                if i-1 >= 0 and int(s[i-1:i+1])%4 == 0:
                    result += i
        # digit 8
        for i in xrange(len(s)):
            if s[i] == '8':
                result += 1
                if i-1 >= 0 and int(s[i-1:i+1])%8 == 0:
                    result += 1
                if i-2 >= 0 and int(s[i-2:i+1])%8 == 0:
                    result += i-1
        # digit 7
        base = 1
        remain = 0
        cnt = [0]*7
        for i in xrange(len(s)):
            remain = (remain+base*(ord(s[~i])-ord('0')))%7
            result += cnt[remain]
            if s[~i] == '7':
                result += 1
                cnt[remain] += 1
            base = (base*10)%7
        return result


# Time:  O(d * n)
# Space: O(d)
# case works, math, freq table
class Solution2(object):
    def countSubstrings(self, s):
        """
        :type s: str
        :rtype: int
        """
        result = 0
        # digit 4
        for i in xrange(len(s)):
            if s[i] == '4':
                result += 1
                if i-1 >= 0 and int(s[i-1:i+1])%4 == 0:
                    result += i
        # digit 8
        for i in xrange(len(s)):
            if s[i] == '8':
                result += 1
                if i-1 >= 0 and int(s[i-1:i+1])%8 == 0:
                    result += 1
                if i-2 >= 0 and int(s[i-2:i+1])%8 == 0:
                    result += i-1
        for d in xrange(1, 9+1):
            if d in (4, 8):
                continue
            base = 1
            remain = 0
            cnt = [0]*d
            c = 0
            for i in xrange(len(s)):
                remain = (remain+base*(ord(s[~i])-ord('0')))%d
                c += cnt[remain]
                if s[~i] == str(d):
                    c += d != 8
                    cnt[remain] += 1
                base = (base*10)%d
            result += c
        return result


# Time:  O(d^2 * n)
# Space: O(d^2)
# dp
class Solution3(object):
    def countSubstrings(self, s):
        """
        :type s: str
        :rtype: int
        """
        result = 0
        dp = [[0]*10 for _ in xrange(10)]
        for i in xrange(1, len(s)+1):
            new_dp = [[0]*10 for _ in xrange(10)]
            x = ord(s[i-1])-ord('0')
            for d in xrange(1, 9+1):
                new_dp[d][x%d] += 1
                for r in xrange(d):
                    new_dp[d][(r*10+x)%d] += dp[d][r]
            dp = new_dp
            result += dp[x][0]
        return result