LeetCode 114. Flatten Binary Tree to Linked List

114. Flatten Binary Tree to Linked List

Difficulty:: Medium

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Solution

Language: Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public void flatten(TreeNode root) {
        helper(root);
    }
    
    private TreeNode[] helper(TreeNode root) {
        if (root == null) {
            return null;
        }
        if (root.left == null && root.right == null) {
            return new TreeNode[]{root, root};
        }
        TreeNode[] leftRes = helper(root.left);
        TreeNode[] rightRes = helper(root.right);
        root.left = null;
        TreeNode right = root;
        if (leftRes != null) {
            right.right = leftRes[0];
            right = leftRes[1];
            right.right = null;
        }
        if (rightRes != null) {
            right.right = rightRes[0];
            right = rightRes[1];
            right.right = null;
        }
        return new TreeNode[] {root, right};
    }
}