# 150 Evaluate Reverse Polish Notation

## 150. [Evaluate Reverse Polish Notation](https://leetcode.com/problems/evaluate-reverse-polish-notation/description/)

## 1. Question

Evaluate the value of an arithmetic expression in [Reverse Polish Notation](https://en.wikipedia.org/wiki/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

```java
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)