/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function invertTree(root: TreeNode | null): TreeNode | null {
    let temp: TreeNode | null;

    if (!root) return null;

    if (root.left || root.right) {
        temp = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(temp);
    }

    return root;
}