226.Invert Binary Tree

Invert a binary tree.

Example:

Input:

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

Output:

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

解题思路：用递归，一层一层交换

/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
function invertTree(root) {
    if (root) {
        [root.left, root.right] = [invertTree(root.right), invertTree(root.left)];
    }
    return root;
}