91. Decode Ways

我的笨方法和取巧方法

class Solution:
    def numDecodings(self, s: str) -> int:
        if not s or s[0] == "0":
            return 0
        n = len(s)
        f = [0] * (n + 1)
        f[0] = 1
        f[1] = 1
        for i in range(2, n+1):
            if s[i-2] == "0" and s[i-1] == "0":
                return 0
            elif s[i-2] != "0" and int(s[i-2:i]) <= 26:
                f[i] = f[i-1] * (not s[i-1] == "0") + f[i-2]
            elif int(s[i-2:i]) > 26 and s[i-1] == "0":
                f[i] = 0
            else:
                f[i] = f[i-1]
        print(f)
        return f[-1]

class Solution:
    def numDecodings(self, s: str) -> int:
        if not s or s[0] == "0":
            return 0
        n = len(s)
        f = [0] * (n + 1)
        f[0] = 1
        f[1] = 1
        for i in range(2, n+1):
            first = int(s[i-1])
            second = int(s[i-2:i])
            if 1 <= first <= 26:
                f[i] += f[i-1]
            if 10 <= second <= 26:
                f[i] += f[i-2]
        return f[-1]