150 Evaluate Reverse Polish Notation

来源：Leetcode

原帖：https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/

题目：
Evaluate the value of an arithmetic expression in Reverse Polish Notation.
(http://en.wikipedia.org/wiki/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

代码：

 class Solution {  
 public:  
   int evalRPN(vector<string> &tokens) {  
     stack<int> s;  
     for (int i = 0; i < tokens.size(); ++i) {  
       if (tokens[i] != "+" && tokens[i] != "-" && tokens[i] != "*" && tokens[i] != "/") {  
         s.push(stoi(tokens[i])); // string to int          
       } else {  
         int op2 = s.top();s.pop();  
         int op1 = s.top();s.pop();  
         int result = 0;          
         if (tokens[i] == "+") {  
           result = op1 + op2;            
         } else if (tokens[i] == "-") {  
           result = op1 - op2;            
         } else if (tokens[i] == "*") {  
           result = op1 * op2;            
         } else if (tokens[i] == "/") {  
           result = op1 / op2;            
         }  
         s.push(result);  
       }  
     }  
     return s.top();  
   }  
 };