LeetCode-in-Java

114. Flatten Binary Tree to Linked List

Medium

Given the root of a binary tree, flatten the tree into a “linked list”:

• The “linked list” should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
• The “linked list” should be in the same order as a pre-order traversal of the binary tree.

Example 1:

Input: root = [1,2,5,3,4,null,6]

Output: [1,null,2,null,3,null,4,null,5,null,6]

Example 2:

Input: root = []

Output: []

Example 3:

Input: root = [0]

Output: [0]

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -100 <= Node.val <= 100

Follow up: Can you flatten the tree in-place (with O(1) extra space)?

To solve the “Flatten Binary Tree to Linked List” problem in Java with a Solution class, we’ll use a recursive approach. Below are the steps:

1. Create a Solution class: Define a class named Solution to encapsulate our solution methods.

2. Create a flatten method: This method takes the root node of the binary tree as input and flattens the tree into a linked list using preorder traversal.

3. Check for null root: Check if the root is null. If so, there’s no tree to flatten, so return.

4. Recursively flatten the tree: Define a recursive helper method flattenTree to perform the flattening.
   • The method should take the current node as input.
   • Perform a preorder traversal of the tree.
   • For each node, if it has a left child:
     • Find the rightmost node in the left subtree.
     • Attach the right subtree of the current node to the right of the rightmost node.
     • Move the left subtree to the right subtree position.
     • Set the left child of the current node to null.
   • Recursively call the method for the right child.
5. Call the helper method: Call the flattenTree method with the root node.

Here’s the Java implementation:

class Solution {
    public void flatten(TreeNode root) {
        if (root == null) return; // Check for empty tree
        flattenTree(root); // Flatten the tree
    }
    
    // Recursive helper method to flatten the tree
    private void flattenTree(TreeNode node) {
        if (node == null) return;
        
        // Flatten left subtree
        flattenTree(node.left);
        
        // Flatten right subtree
        flattenTree(node.right);
        
        // Save right subtree
        TreeNode rightSubtree = node.right;
        
        // Attach left subtree to the right of the current node
        node.right = node.left;
        
        // Set left child to null
        node.left = null;
        
        // Move to the rightmost node of the flattened left subtree
        TreeNode current = node;
        while (current.right != null) {
            current = current.right;
        }
        
        // Attach the saved right subtree to the right of the rightmost node
        current.right = rightSubtree;
    }
    
    // TreeNode definition
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        
        TreeNode() {}
        TreeNode(int val) { this.val = val; }
        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }
}

This implementation follows the steps outlined above and efficiently flattens the binary tree into a linked list using preorder traversal in Java.

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