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Decode the Coding Interview in Java: Real-World Examples

Flatten Binary Tree to Linked List

Explore how to transform a given binary tree into a linked list using pre-order traversal. Understand and apply an efficient algorithm to modify the tree in place with O(n) time and O(1) space complexity, preparing you to solve similar tree manipulation problems in coding interviews.

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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 null, 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

Java
class Solution {
  public static TreeNode flatten(TreeNode root) {
    // Write your code here
    return root;
  }
}
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 ...