leetcode面试题2：Evaluate Reverse Polish Notation

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题目如下：

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

意思是实现逆波兰表达式，思路是利用栈，当表达式是数字时，将数字压入栈，当表达式是运算符时，从栈中取出2个数字进行运算，将结果压入栈顶。

AC代码如下：

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

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