Invert Binary Tree

Invert a binary tree.

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

to

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

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

============

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode root) {
        Stack<TreeNode> stk = new Stack<TreeNode>();
        stk.push(root);
        while (!stk.isEmpty()) {
            TreeNode node = stk.pop();
            if (node != null) {
                TreeNode left = node.left;
                TreeNode right = node.right;
                node.left = right;
                node.right = left;
                stk.push(left);
                stk.push(right);
            }
        }
        return root;
    }
}