150 Evaluate Reverse Polish Notation

150. Evaluate Reverse Polish Notation

1. Question

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

2. Implementation

思路: 当遇到运算符时，要处理前面的数字，所以用stack

public class Solution {
    public int evalRPN(String[] tokens) {
        int num1 = 0, num2 = 0;   
        Stack<Integer> stack = new Stack<>();

        for (String token : tokens) {
            switch(token) {
                case "+":
                    num1 = stack.pop();
                    num2 = stack.pop();
                    stack.push(num1 + num2);
                    break;
                case "-":
                    num1 = stack.pop();
                    num2 = stack.pop();
                    stack.push(num2 - num1);
                    break;
                case "*":
                    num1 = stack.pop();
                    num2 = stack.pop();
                    stack.push(num1 * num2);
                    break;
                case "/":
                    num1 = stack.pop();
                    num2 = stack.pop();
                    stack.push(num2 / num1);
                    break;
                default:
                    stack.push(Integer.parseInt(token));

            }
        }
        return stack.peek();
    }
}

3. Time & Space Complexity

时间和空间都是O(n)