Solution.java

/**
 * 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;
        }
        List<TreeNode> list = new ArrayList<TreeNode>();
        list.add(root);
        while (list.size() > 0) {
            TreeNode node = list.get(0);
            list.remove(node);
            TreeNode tempNode = node.left;
            node.left = node.right;
            node.right = tempNode;
            if (node.left != null) {
                list.add(node.left);
            }
            if (node.right != null) {
                list.add(node.right);
            }
        }
        return root;
    }
}