# # 【LeetCode】91. Decode Ways 解题报告（Python）

## # 题目描述：

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).
``````

## # 解题方法

``````dp[i] = dp[i-1] if s[i] != '0'
+ dp[i-2] if '9' < s[i-2:i] < '27'
``````

``````class Solution(object):
def numDecodings(self, s):
"""
:type s: str
:rtype: int
"""
dp = [0] * (len(s) + 1)
dp[0] = 1
for i in range(1, len(dp)):
if s[i-1] != '0':
dp[i] = dp[i-1]
if i != 1 and '09' < s[i-2:i] < '27':
dp[i] += dp[i-2]
return dp[-1]
``````

1. https://leetcode.com/problems/decode-ways/discuss/163707/Python-From-O(N)-Space-To-O(1)-Space-Solutions

## # 日期

2018 年 8 月 27 日 ———— 就要开学了！