Invert Binary Tree

Invert a binary tree.

Example:

Input:

1
2
3
4
5
     4
   /   \
  2     7
 / \   / \
1   3 6   9

Output:

1
2
3
4
5
     4
   /   \
  7     2
 / \   / \
9   6 3   1

Trivia: This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.

Solution:

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/**
 * Definition for a binary tree node.
 * 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;
 *     }
 * }
 */
class Solution {
    
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            TreeNode tmp = node.left;
            node.left = node.right;
            node.right = tmp;
            if (node.left !=  null) queue.add(node.left);
            if (node.right != null) queue.add(node.right);
        }
        return root;
    }
    
    public TreeNode invertTree2(TreeNode root) {
        if (root == null) {
            return null;
        }
        TreeNode left = invertTree2(root.left);
        TreeNode right = invertTree2(root.right);
        root.left = right;
        root.right = left;
        return root;
    }
}