leetcode[91]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.

class Solution {
public:
int check(char  one)
{
    int tmp=one-'0';
    return (tmp>=1&&tmp<=9)?1:0;
//    return one=='0'?0:1;
}
int check(char one, char two)
{
    int tmp=10*(one-'0')+two-'0';
    return (tmp>=10&&tmp<=26)?1:0;
//    return (one=='1'||(one=='2'&&two<='6'))?1:0;
}
int numDecodings(string s) 
{
    if(s.empty())return 0;
    int len=s.length();
    vector<int> f(len,0);
    if(len==1)
    {
        f[0]=check(s[0]);
        return f[0];
    }
    f[0]=check(s[0]);
    f[1]=(check(s[0])&&check(s[1]))+check(s[0],s[1]);
    for (int i=2;i<len;i++)
    {
        if(check(s[i]))f[i]+=f[i-1];
        if(check(s[i-1],s[i]))f[i]+=f[i-2];
    }
    return f[len-1];
}
};