Evaluate reverse polish notation

/*
Logic:
push numbers into stack and whenever operators comes pop top two items
and operate on them and push the result.
*/
long long int _stoi(string s){
    bool sign=false;
    long long int ans=0;
    int i=0;
    if(s[0]=='-'){sign=true; i++;}
    while(i<s.size()){
        ans=ans*10+(s[i]-'0');
        i++;
    }
    if(sign) return -ans;
    return ans;
}
class Solution {
public:
    int evalRPN(vector<string>& arr) {
        stack<long long int> st;
        for(auto itr:arr){
            if(itr=="+" || itr=="-" || itr=="*" || itr=="/"){
                long long int b=st.top(); st.pop();
                long long int a=st.top();st.pop();
                if(itr=="+") st.push(a+b);
                else if(itr=="-") st.push(a-b);
                else if(itr=="*") st.push(a*b);
                else st.push(a/b);
            }
            else st.push(_stoi(itr));
        }
        return st.top();
    }
};