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();

    }

};