[LeetCode] Decode Ways Problem

class Solution {
public:
    bool isValid(string s) {

        if(s[0]=='0'){
            return false;
        }
        if(stoi(s)>26) {
            return false;
        }
        return true;
    }

    int f(string s, int currentSplit,vector<int> dp){

        if(dp[currentSplit]!=-1) {
            return dp[currentSplit];
        }

        if(currentSplit==0) { return 1;}
        if(currentSplit<0) { return 0;}

        int ans=0;

        if(currentSplit>=1 && isValid(s.substr(currentSplit-1,1))){
            ans+=f(s,currentSplit-1,dp);
        }
        if(currentSplit>=2 && isValid(s.substr(currentSplit-2,2))){
            ans+=f(s,currentSplit-2,dp);
        }
        return dp[currentSplit]=ans;




    }
    int numDecodings(string s) {

         vector<int>dp(s.length()+1,-1);
         return f(s,s.length(),dp);
    }
};

//at each position- we can try split or not split