Leetcode: 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：Reverse Polish Notation可以通过post traversal计算树来得到，我们可以通过栈来evaluate它的值，代码如下：
 1 class Solution {
 2 public:
 3     int evalRPN(vector<string> &tokens) {
 4         int result = 0;
 5         stack<int> S;
 6         
 7         for(int i = 0; i < tokens.size(); i++){
 8             if(is_operator(tokens[i])){
 9                 int rv = S.top();
10                 S.pop();
11                 int lv = S.top();
12                 S.pop();
13                 if(tokens[i] == "+")
14                     S.push(lv + rv);
15                 else if(tokens[i] == "-")
16                     S.push(lv - rv);
17                 else if(tokens[i] == "*")
18                     S.push(lv * rv);
19                 else if(tokens[i] == "/")
20                     S.push(lv / rv);
21             }else{
22                 S.push(atoi(tokens[i].c_str()));
23             }
24         }
25         return S.top();
26     }
27     
28     bool is_operator(string s){
29         return s == "+" || s == "-" || s == "*" || s == "/";
30     }
31 };

 








    
posted on 
2014-12-15 21:30 
Ryan-Xing 
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