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"

The number of ways decoding "12" is 2.

1. 看別人的 （這個我不會）

class Solution {
public:
    bool isValid(char c){
        return c <= '9' && c >'0';
    }
    bool isvalid(char c1, char c2){
        if(c1 == '1')
            return c2 <='9' && c2 >='0';
        if(c1 == '2')
            return c2 <='6' && c2 >='0';
        return false;
    }
    int numDecodings(string s) {
        int len = s.size();
        if(len <= 0)
        return 0;
        vector<int>res(len+1,0);
        res[0] = 1;
        for(int i = 0; i < len; i++){
            if(isValid(s[i]))
                res[i+1] += res[i];
            if(isvalid(s[i-1],s[i]))
                res[i+1] += res[i-1];
        }
        return res[len];
    }
};