leetcode: 91. Decode Ways

# 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.

class Solution():
    def numDecodings(self, s):
        if len(s) == 0 or s[0] == '0':
            return 0
        pre, pre_pre = 1, 0
        for i in range(len(s)):
            cur = 0
            if s[i] != '0':
                cur = pre
            if i > 0 and (s[i - 1] == '1' or (s[i - 1] == '2' and s[i] <= '6')):
                cur += pre_pre
            pre, pre_pre = cur, pre
        return pre


if __name__ == "__main__":
    assert list(map(lambda x: Solution().numDecodings(x), ["0", "10", "10", "103", "1032", "10323"])) == [0, 1, 1, 1, 1, 2]