Invert Binary Tree

Problem

Invert a binary tree.

Example

      1         1
     / \       / \
    2   3  => 3   2
       /       \
      4         4

Challenge

Do it in recursion is acceptable, can you do it without recursion?

Code - Java

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *   public int val;
 *   public TreeNode left, right;
 *   public TreeNode(int val) {
 *     this.val = val;
 *     this.left = this.right = null;
 *   }
 * }
 */
public class Solution {
    /**
     * @param root: a TreeNode, the root of the binary tree
     * @return: nothing
     */
    public void invertBinaryTree(TreeNode root) {
        if (root == null) {
            return;
        }
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        invertBinaryTree(root.left);
        invertBinaryTree(root.right);
    }
}