226. Invert Binary Tree

/**
 * 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;
        }

        // swap the address of left tree and right tree of the current node
        TreeNode temp = root.right;
        root.right = root.left;
        root.left = temp;

        // the recursively call of the left tree
        invertTree(root.left);

        // the recursively call of the right tree
        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; }
 * }
 */
class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.addFirst(root);
        while (!queue.isEmpty()) {
            TreeNode x = queue.removeFirst();
            
            // swap the address of left tree and right tree of the current node
            TreeNode temp = x.left;
            x.left = x.right;
            x.right = temp;
            
            if (x.left != null) {
                queue.addLast(x.left);
            }
            if (x.right != null) {
                queue.addLast(x.right);
            }
        }
        return root;
    }
}