LeetCode: Evaluate Reverse Polish Notation

题目: Evaluate Reverse Polish Notation

Evaluatethe 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

简单题，借助一个stack就可以实现，将数值入栈，遇操作符时将两个数值出栈，计算后再入栈，如此即可实现。

int evalRPN(vector<string> &tokens)

{

    stack<int> stokens;

    for (int i = 0; i < tokens.size(); ++ i) {

        if (tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/") {

            int x1 = stokens.top();

            stokens.pop();

            int x2 = stokens.top();

            stokens.pop();

            int x3 = caculate(x1, x2, tokens[i]); //计算之后再入栈

            stokens.push(x3);

        }

        else {

            stokens.push(atoi(tokens[i].c_str()));

        }

    }

    return stokens.top();

}

int caculate(int x1, int x2, string s) 

{

    int result;

    if (s == "+")

        result = x1 + x2;

    else if (s == "-") 

        result = x1 - x2;

    else if (s == "*")

        result = x1 * x2;

    else {

        if (x1 >= x2)

            result = x1 / x2;

        else

            result = x2 / x1;

    }

    return result;

}