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 a non-empty string containing only digits, determine the total number of ways to decode it.

Example 1:
Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).

Example 2:
Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

分析：
有思路，但是脑子不够灵活，自己写得代码有问题。
借鉴别人的。
和斐波那契数列有点像，或者变态青蛙跳台阶。但是有限制条件。比如不能为0，不能大于26。
code：

def numDecodings(s):
    if s[0] == "0":
        return 0
    n = len(s)
    ans = [0 for i in range(n+1)]   # 后一个位置存储当前位置的解码数目
    ans[0] = ans[1] = 1
    for i in range(1, n):
        one = int(s[i])
        two = int(s[i-1:i+1])
        if one > 0:
            ans[i+1] = ans[i]
        if 10 <= two <= 26:
            ans[i+1] += ans[i-1]
    return ans[n]