CODEWITHSUNDEEP
python
yantiz
yantiz

Reputation: 97

What is the reason that I have to use another variable name to have my code run?

I am a python newbie -.- The following code is what I write on leetcode:

def postorderTraversal(self, root):
        """
        :type root: TreeNode
        :rtype: List[int]
        """
        result, stack, current, last_traversed = [], [], root, None
        while stack or current:
            if current:
                stack.append(current)
                current = current.left
            else:
                parent = stack[-1]
                if parent.right in (None, last_traversed):
                    result.append(parent.val)
                    last_traversed = stack.pop()
                else:
                    current = parent.right
        return result

It works apparently, but I used to replace all parent with current and the program gives me the result of "Time Limit Exceeded".

What I was wondering is why I can't simply use current throughout the code. Why I have to create another variable for the parent node?

Upvotes: 1

Views: 57

Answers (2)

jimifiki
jimifiki

Reputation: 5534

If stack is true, current is False and stack[-1].right is one among None and last_traversed you don't want current to be modified.

This is a question about the algorithm, not about the language.

Upvotes: 1

Tom Zych
Tom Zych

Reputation: 13576

In the branch that begins with if parent.right, parent is set but current is not. So if you replace it with current the behavior is different.

Upvotes: 2

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