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 evalRPN(vector<string> &tokens) {
        stack<int> s_opd;
        int lopt = 0;
        int ropt = 0;
        for(int i = 0; i < tokens.size(); i++) {
            if(tokens[i] == "+"){
                // pick out the first number
                lopt = s_opd.top();
                s_opd.pop();
                // pick out the second number
                ropt = s_opd.top();
                s_opd.pop();
                // push the result
                s_opd.push(ropt + lopt);
            } else if (tokens[i] == "-") {
                lopt = s_opd.top();
                s_opd.pop();
                ropt = s_opd.top();
                s_opd.pop();
                s_opd.push(ropt - lopt);
                
            }else if(tokens[i] == "*") {
                lopt = s_opd.top();
                s_opd.pop();
                ropt = s_opd.top();
                s_opd.pop();
                s_opd.push(ropt * lopt);
                
            }else if(tokens[i] == "/") {
                lopt = s_opd.top();
                s_opd.pop();
                ropt = s_opd.top();
                s_opd.pop();
                if(lopt == 0) {
                    s_opd.push(0);
                } else {
                     s_opd.push(ropt / lopt);
                }
            } else {
                s_opd.push( atoi(tokens[i].c_str()) );
            }
        }
        return s_opd.top();
    }
};

注意：

1. STL stack 的pop不会返回数值，只会消除一个数据。

2.运算符 / 要考虑分母为0的情况。