150 Evaluate Reverse Polish Notation

链接：https://leetcode.com/problems/evaluate-reverse-polish-notation/
问题描述：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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这个问题就是计算表达式的简单版本，因为数据和运算符已经直接给出，假如给出一个字符串那么难度会更高。题目解题思路如下，每当遇到数字则压栈，遇到运算符则取栈顶两个元素进行运算，运算结果压栈。

class Solution {public:    int evalRPN(vector<string>& tokens) {        stack<int> data;        int k1,k2;        for(int i=0;i<tokens.size();i++)        {           if(tokens[i]=="/")           {               k1=data.top();               data.pop();               k2=data.top();               data.pop();               data.push(k2/k1);               continue;           }           else if(tokens[i]=="*")           {               k1=data.top();               data.pop();               k2=data.top();               data.pop();               data.push(k2*k1);               continue;           }           else if(tokens[i]=="+")           {               k1=data.top();               data.pop();               k2=data.top();               data.pop();               data.push(k2+k1);               continue;           }           else if(tokens[i]=="-")           {               k1=data.top();               data.pop();               k2=data.top();               data.pop();               data.push(k2-k1);               continue;           }           data.push(atoi(tokens[i].c_str()));        }        return data.top();    }};