Leetcode 91 Decode Ways

'A' -> 1
'B' -> 2
...
'Z' -> 26

class Solution {
public:
    int numDecodings(string s) {
        if(s.empty()) return 0;
        vector<int> dp(s.size()+1,0);
        dp[0]=1;
        if(s[0]!='0') dp[1]=1;
        for(int i=1;i<s.size();i++)
        {
            if(s[i]!='0') dp[i+1]+=dp[i];
            if((s[i]>='0' && s[i]<='9' && s[i-1]=='1') || (s[i-1]=='2' && s[i]>='0' && s[i]<='6')) dp[i+1]+=dp[i-1];
        }
        return dp[s.size()];
    }
};