Invert a binary tree.

Example:

Input:

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

Output:

     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.

Solution1

package question226;

import java.util.LinkedList;
import java.util.List;

public class Solution2 {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;

        List<TreeNode> nodeList = new LinkedList<>();
        nodeList.add(root);
        while (!nodeList.isEmpty()) {
            int size = nodeList.size();
            for (int i = 0; i < size; i++) {
                TreeNode curNode = nodeList.remove(0);
                TreeNode tempNode = curNode.left;
                curNode.left = curNode.right;
                curNode.right = tempNode;
                if (curNode.left != null) {
                    nodeList.add(curNode.left);
                }
                if (curNode.right != null) {
                    nodeList.add(curNode.right);
                }
            }
        }
        return root;
    }
}

Solution2

package question226;

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return null;
       
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        
        root.left = right;
        root.right = left;
        return root;
        
    }
}