> For the complete documentation index, see [llms.txt](https://r24zeng.gitbook.io/leetcode-notebook/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://r24zeng.gitbook.io/leetcode-notebook/bu-chong-2140-dao/150.-evaluate-reverse-polish-notation.md).

# 150. Evaluate Reverse Polish Notation

{% hint style="info" %}
Not hard if know the principle behind and coding as the process of human solving

Key idea: when see notation, compute the two numbers before it. 

`stack` is perfect to this problem
{% endhint %}

### Solution:

1. traversal all elements of the list
2. if it's a number, push to stack
3. if it's a notation, pop two numbers and compute, then push back to stack

```python
class Solution:
    def evalRPN(self, tokens: List[str]) -> int:      
        i = 0
        stack = []
        while i < len(tokens):
            st = tokens[i]
            if st != '+' and st != '-' and st != '*' and st != '/':
                stack.append(st)
            else:
                num2 = int(stack.pop())
                num1 = int(stack.pop())
                if st == '+':
                    res = num1 + num2
                elif st == '-':
                    res = num1 - num2
                elif st == '*':
                    res = num1 * num2
                else:
                    if num1*num2 < 0:
                        res = num1*(-1)//num2 * (-1)
                    else:
                        res = num1//num2
                stack.append(res)
            i += 1
        
        return stack.pop()
```

{% hint style="danger" %}
如果用python则不能用recursive来做，因为python不能传递reference，意味着函数的参数无法成为全局变量，而导致一直在变
{% endhint %}

Time = $$o(n)$$ , space = $$O(n)$$&#x20;


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