Decode the Coding Interview in Python: Real-World Examples/

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Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List

Understand and solve the interview question "Flatten Binary Tree to Linked List".

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Description

We are given the root of a binary tree. We have to flatten the tree into a linked list. The left child of the linked list is always None, and the right child points to the next node in the list. The nodes in the linked list should be in the same order as the pre-order traversal of the given binary tree.

Constraints

  • -100 <= ‘Node.val’ <= 100
  • The tree contains nodes in the range [0, 2000].

Let’s see how a flattened tree looks like in the illustration below:

Coding exercise

class TreeNode:
   def __init__(self, val=0, left=None, right=None):
    self.val = val
    self.left = left
    self.right = right
def flatten(root):
  
  # Write your code here
  pass
Flatten binary tree to linked list

Solution

We will flatten the given binary tree into the linked list. We will keep the linked list in the pre-order traversal of the binary tree. We will be traversing the tree starting from the root.

Let’s see the following algorithm to implement the described problem.

  1. First, we will implement the loop that keeps going until the current node value becomes None, which would happen when we traverse the entire tree. ...