> For the complete documentation index, see [llms.txt](https://hao-fu-1.gitbook.io/oj/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hao-fu-1.gitbook.io/oj/stack/jie-he-qian-mian-de-shu-ru/evaluate-reverse-polish-notation.md).

# Evaluate Reverse Polish Notation

<https://leetcode.com/problems/evaluate-reverse-polish-notation/description/>

> 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.

## Thoughts

stack的又一经典应用。

## Code

```
class Solution {
    private boolean isNum(String token) {
        if ("+-*/".contains("" + token.charAt(0)) && token.length() == 1) {
            return false;
        }  
        return true;
    } 

    public int evalRPN(String[] tokens) {
        Stack<Integer> stack = new Stack<>();
        for (String str : tokens) {
            if (isNum(str)) {
                stack.push(Integer.parseInt(str));
            } else {
                int op2 = stack.pop(), op1 = stack.pop();
                switch (str) {
                    case "+":
                        stack.push(op1 + op2);
                        break;
                    case "-":
                        stack.push(op1 - op2);
                        break;
                    case "*":
                        stack.push(op1 * op2);
                        break;
                    default:
                        stack.push(op1 / op2);
                }
            }
        }

        return stack.pop();
    }
}
```

## Analysis

做题耗时: 8min

Errors:

1. op1和op2顺序弄反了
2. -和负数没分清

时空复杂度都是O(n).