LeetCode Invert Binary Tree

InvertBinaryTree.java

import java.util.*;

public class InvertBinaryTree {

    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */

    // recursive 300ms
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return root;

        TreeNode tmp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(tmp);

        return root;
    }

    // iterative 304ms
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return root;

        Stack<TreeNode> t = new Stack<>();
        t.push(root);
        while(!t.isEmpty()){
            TreeNode n = t.pop();

            TreeNode tmp = n.left;
            n.left = n.right;
            n.right = tmp;

            if(n.right != null)
                t.push(n.right);
            if(n.left != null)
                t.push(n.left);
        }
        return root;
    }
}