leetcode: 91. Decode Ways

Q

A message containing letters from A-Z is being encoded to numbers using the following mapping:

'A' -> 1'B' -> 2...'Z' -> 26

Given an encoded message containing digits, determine the total number of ways to decode it.

For example,
Given encoded message “12”, it could be decoded as “AB” (1 2) or “L” (12).

The number of ways decoding “12” is 2.

AC

class Solution:    def numDecodings(self, s):        """        :type s: str        :rtype: int        """        if not s:            return 0        self.memo = {}        return self.getCount(s, 0)    def getCount(self, s, i):        if i in self.memo:            return self.memo[i]        if not s:            ret = 1        elif len(s) ==1:            if s!='0':                ret = 1            else:                ret = 0        else:            if s[0]=='0':                ret = 0            else:                ret = self.getCount(s[1:], i+1)                if int(s[:2])<=26:                    ret += self.getCount(s[2:], i+2)        self.memo[i] = ret        return ret# Time:  O(n)# Space: O(1)class Solution2(object):    def numDecodings(self, s):        """        :type s: str        :rtype: int        """        if len(s) == 0 or s[0] == '0':            return 0        prev, prev_prev = 1, 0        for i in xrange(len(s)):            cur = 0            if s[i] != '0':                cur = prev            if i > 0 and (s[i - 1] == '1' or (s[i - 1] == '2' and s[i] <= '6')):                cur += prev_prev            prev, prev_prev = cur, prev        return previf __name__ == "__main__":    assert map(lambda x: Solution().numDecodings(x), ["0", "10", "10", "103", "1032", "10323"]) == [0, 1, 1, 1, 1, 2]

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