# Question

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).
```

# Solution

Use DP from the end to front.

```
class Solution:
def numDecodings(self, s):
"""
:type s: str
:rtype: int
"""
if s[0] == '0':
return 0
p, pp, curr = 1, 0, 0
for i in range(len(s) - 1, -1, -1):
curr = 0 if s[i] == '0' else p
if i < len(s) - 1 and (s[i] == '1' or (s[i] == '2' and s[i + 1] < '7')):
curr += pp
pp = p
p = curr
return curr
```