150. 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

分析：这一题就是写程序计算逆波兰式的结果，遍历表达式，碰到操作数入栈，碰到操作符就从栈顶取出两个操作数，再将计算后的结果入栈，最后栈中剩余的唯一操作数就是计算结果。

class Solution {public:    int evalRPN(vector<string> &tokens) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        int len = tokens.size();        stack<int> S;        for(int i = 0; i < len; i++)        {            if(tokens[i] == "+" || tokens[i] == "-" ||                tokens[i] == "*" || tokens[i] == "/")            {                int op2 = S.top(); S.pop();                int op1 = S.top(); S.pop();                S.push( op(op1, op2, tokens[i][0]) );            }            else                S.push(stoi(tokens[i]));        }        return S.top();    }    int op(int op1, int op2, char optor)    {        if(optor == '+')return op1 + op2;        else if(optor == '-')return op1 - op2;        else if(optor == '*')return op1 * op2;        else return op1 / op2;    }};