150. Evaluate Reverse Polish Notation

Problem:

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Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Analysis:
It's every easy with stack, but need to pay attention of the order of stack pops when doing divide.

Solution:

class Solution {
    public int evalRPN(String[] tokens) {
        Stack<Integer> 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 n2 = stack.pop(), n1 = stack.pop();
                    stack.push(n1 / n2);
                    break;
                default: 
                    stack.push(Integer.valueOf(s));
                    break;
            }
        }
        return stack.peek();
    }
}