leetcode——Evaluate Reverse Polish Notation 求算式值（AC）

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> myStack;
        int first,second,merge;
        for(int i=0; i<tokens.size(); i++)
        {
            if(tokens[i]=="+" || tokens[i]=="-" || tokens[i]=="*" || tokens[i]=="/")
            {
                first = myStack.top();
                myStack.pop();
                second = myStack.top();
                myStack.pop();
                if(tokens[i] == "+")
                    merge = first + second;
                else if(tokens[i] == "-")
                    merge = second - first;
                else if(tokens[i] == "*")
                    merge = first * second;
                else if(tokens[i] == "/")
                    merge = second / first;
            }
            else
            {
                merge = atoi(tokens[i].c_str());
            }
            myStack.push(merge);
        }
        return myStack.top();
    }
};