[leetcode] Evaluate Reverse Polish Notation

From : https://leetcode.com/problems/evaluate-reverse-polish-notation/

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

class Solution {
public:
    int resolve(string s) {
        int num=0, size=s.size(), flag=1;
        for(int i=0; i<size; i++) {
            if(s[i]=='-') flag=-1;
            if(s[i]>='0' && s[i]<='9') {
                num = num*10 + s[i]-'0';
            }
        }
        return flag*num;
    }
    int evalRPN(vector<string>& tokens) {
        string cur;
        stack<int> nums;
        int size=tokens.size();
        for(int i=0; i<size; i++) {
            cur = tokens[i];
            if(cur=="+" || cur=="-" || cur=="*" || cur=="/") { ////b . a
                int a = nums.top();
                nums.pop();
                int b = nums.top();
                nums.pop();
                if(cur == "+") {
                    nums.push(b+a);
                } else if(cur == "-") {
                    nums.push(b-a);
                } else if(cur == "*") {
                    nums.push(b*a);
                } else /*if(a!=0)*/{
                    nums.push(b/a);
                }
            } else {
                nums.push(resolve(cur));
            }
        }
        return nums.top();
    }
};