[Leetcode]Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation
Total Accepted: 55506 Total Submissions: 249113 Difficulty: Medium

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

[“2”, “1”, “+”, “3”, ““] -> ((2 + 1) 3) -> 9
[“4”, “13”, “5”, “/”, “+”] -> (4 + (13 / 5)) -> 6

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根据操作符不同进行不同的操作，之后把运算结果重新压入栈中即可！

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        int len  = tokens.size();
        if(!len)    return 0;
        stack<string> s;
        int i = 0;
        while(i <= len - 1){
            if(!isOperator(tokens[i])){
                s.push(tokens[i]);
                ++i;
            }
            else if(isOperator(tokens[i])){
                int r = stoi(s.top());
                s.pop();
                int l = stoi(s.top());
                s.pop();
                int res;
                if(tokens[i] == "+"){
                    res = l + r;
                }
                else if(tokens[i] == "-"){
                    res = l - r;
                }
                else if(tokens[i] == "*"){
                    res = l * r;
                }
                else if(tokens[i] == "/"){
                    res = l / r;
                }
                s.push(to_string(res));
                ++i;
            }
        }
        return stoi(s.top());
    }
private:
    bool isOperator(string s){
        if(s == "+" || s == "-" || s == "*" || s == "/"){
           return true; 
        }
        else return false;
    }
};