LeetCode算法题目：Evaluate Reverse Polish Notation

题目：

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

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分析：

逆波兰式计算等式非常容易，利用堆栈即可完成。另外，程序中还需要实现整数与字符串之间的相互转换。

这题目以前做过，所以一遍AC。

代码：

class Solution {public:    int evalRPN(vector<string>& tokens) {        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]));//stoi() 把字符串转换成数字        }        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;    }};