LeetCode----Decode Ways

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.

分析：

动态规划题，注意，输入会有很多不合法的情况，比如‘100’之类的，这种情况下decoding方式为0。

不合法的情况有：串长度为0；串以0开头；串中出现00；串中出现30，40，50等串。

代码：

class Solution(object):
    def numDecodings(self, s):
        """
        :type s: str
        :rtype: int
        """
        f = {-1: 1}
        s_len = len(s)
        if s_len == 0 or s[0] == '0':
            return 0
        f[0] = 1
        i = 1
        while i < s_len:
            if s[i] == '0' and (s[i - 1] > '2' or s[i - 1] == '0'):
                return 0

            if s[i] == '0':
                f[i] = f[i - 2]
            elif 10 <= int(s[i-1:i+1]) <= 26:
                f[i] = f[i - 1] + f[i - 2]
            else:
                f[i] = f[i - 1]
            i += 1
        return f[s_len - 1]