150. Evaluate Reverse Polish Notation

Description

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

Solution

Stack, time O(n), space O(n)

原来String也可以switch case的啊。。比if else简洁多啦。

class Solution {
    public int evalRPN(String[] tokens) {
        if (tokens == null || tokens.length == 0) {
            return 0;
        }
        
        Stack stack = new Stack<>();
        
        for (String s : tokens) {
            switch (s) {
                case "+":
                    stack.push(stack.pop() + stack.pop());
                    break;
                case "-":
                    stack.push(-stack.pop() + stack.pop());
                    break;
                case "*":
                    stack.push(stack.pop() * stack.pop());
                    break;
                case "/":
                    int a = stack.pop();
                    stack.push(stack.pop() / a);
                    break;
                default:
                    stack.push(Integer.parseInt(s));
                    break;
            }
        }

        return stack.pop();
    }
}