学步园

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.

字符串处理，试着逆序，没做成。只能顺序了

class Solution {
public:
    int numDecodings(string s) {
        if(s.empty())
            return 0;
        int len = s.length();
        int f[len];

        for(int i = 0; i < len; i++) {
            if(s[i] < '0' || s[i] > '9')
                return 0;
            else if(s[i] == '0'){
                if(i == 0 || s[i-1] == '0' || s[i-1] > '2')
                    return 0;
                else f[i] = i > 1 ? f[i-2] : 1;
            }
            else {  
               if(i > 0 && s[i-1] == '1' || s[i-1] == '2' && s[i] <= '6' )
                    f[i] = (i > 0 ? f[i-1] : 1) + (i > 1 ? f[i-2] : 1);
                else 
                    f[i] = i > 0 ? f[i-1] : 1; 
                
            }  
         }
            return f[len-1];
    }
};