[leetcode]Decode Ways

class Solution {//dppublic:    int numDecodings(string s) {        int len=s.length();        if(len<1){            return 0;        }        vector<int> f(len,0);//f[i]表示到第i个字符的decoding ways        for(int i=0;i<len;++i){            if(i==0){                if(s[i]>='1'&&s[i]<='9'){                    f[0]=1;                }            }else{//i>=1                string tmp(s,i-1,2);//截取s[i-1]s[i]                if(tmp>="10"&&tmp<="26"){                    if(i>1){                        f[i]+=f[i-2];                    }else{//i==1,没有i - 2这个index                        f[i]+=1;                    }                }                if(s[i]>='1'&&s[i]<='9'){                    f[i]+=f[i-1];                }            }        }        return f[len-1];    }};