Evaluate Reverse Polish Notation

题目：

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

解答：

 1 public class Solution {
 2     
 3     private static final Set<String> OPERATORS = 
 4         new HashSet<>(Arrays.asList("+", "-", "*", "/"));
 5 
 6     public int eval(int x, int y, String operator) {
 7         switch(operator) {
 8             case "+":
 9                 return x+y;
10             case "-":
11                 return x-y;
12             case "*":
13                 return x*y;
14             case "/":
15                 return x/y;
16         }
17     }
18 
19     public int evalPRN(String[] tokens) {
20         Stack<Integer> stack = new Stack<>();
21         for(String token : tokens) {
22             if(OPERATORS.contais(token)) {
23                 int y = stack.pop();
24                 int x = stack.pop();
25                 stack.push(eval(x, y, token));
26             } else {
27                 stack.push(Integer.parseInt(token));
28             }
29         }
30 
31         return stack.pop();
32     }
33 }