[LeetCode] 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.

DP解决

 1 class Solution {
 2 private:
 3     vector<int> f;
 4 public:
 5     int getF(int index)
 6     {
 7         if (index < 0)
 8             return 1;
 9         else
10             return f[index];
11     }
12     
13     int numDecodings(string s) {
14         // Start typing your C/C++ solution below
15         // DO NOT write int main() function
16         if (s.size() == 0)
17             return 0;
18             
19         f.resize(s.size());
20         
21         for(int i = 0; i < s.size(); i++)
22         {
23             f[i] = 0;
24             if (i >= 1)
25             {
26                 string a(s, i - 1, 2);
27                 if ("10" <= a && a <= "26")
28                     f[i] += getF(i-2);
29                 if ('1' <= s[i] && s[i] <= '9')
30                     f[i] += getF(i-1);
31             }
32             else
33             {
34                 if ('1' <= s[i] && s[i] <= '9')
35                     f[i] = 1;
36             }
37         }
38         
39         return f[f.size() - 1];
40     }
41 };