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

思路：
逆波兰表达式，使用一个栈来计算。
代码：

 1     int evalRPN(vector<string> &tokens) {
 2         // IMPORTANT: Please reset any member data you declared, as
 3         // the same Solution instance will be reused for each test case.
 4         int l = tokens.size();
 5         if(l == 0)
 6             return 0;
 7         stack<int> nums;
 8         for(int i = 0; i < l; i++){
 9             string tmp = tokens[i];
10             if(tmp == "+"){
11                 int num1 = nums.top();
12                 nums.pop();
13                 int num2 = nums.top();
14                 nums.pop();
15                 nums.push(num1 + num2);
16             }
17             else if(tmp == "-"){
18                 int num1 = nums.top();
19                 nums.pop();
20                 int num2 = nums.top();
21                 nums.pop();
22                 nums.push(num2 - num1);
23             }
24             else if(tmp == "*"){
25                 int num1 = nums.top();
26                 nums.pop();
27                 int num2 = nums.top();
28                 nums.pop();
29                 nums.push(num1 * num2);
30             }
31             else if(tmp == "/"){
32                 int num1 = nums.top();
33                 nums.pop();
34                 int num2 = nums.top();
35                 nums.pop();
36                 nums.push(num2 / num1);
37             }
38             else
39                 nums.push(atoi(tmp.c_str()));
40         }
41         return nums.top();
42     }