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：

public class Solution {
    
    private static final Set<String> OPERATORS = 
        new HashSet<>(Arrays.asList("+", "-", "*", "/"));

    public int eval(int x, int y, String operator) {
        switch(operator) {
            case "+":
                return x+y;
            case "-":
                return x-y;
            case "*":
                return x*y;
            case "/":
                return x/y;
        }
    }

    public int evalPRN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for(String token : tokens) {
            if(OPERATORS.contais(token)) {
                int y = stack.pop();
                int x = stack.pop();
                stack.push(eval(x, y, token));
            } else {
                stack.push(Integer.parseInt(token));
            }
        }

        return stack.pop();
    }
}