LC.226.Invert Binary Tree

https://leetcode.com/problems/invert-binary-tree/description/
Invert a binary tree.
     4
   /   \
  2     7
 / \   / \
1   3 6   9
to
     4
   /   \
  7     2
 / \   / \
9   6 3   1
time:  o(n) : n nodes 
space: o(n): worst case linkedlist and n calling stack 

public TreeNode invertTree(TreeNode root) {
        if (root == null) return null ;
        TreeNode left = invertTree(root.left) ;
        TreeNode right = invertTree(root.right) ;
        root.right = left ;
        root.left = right ;
        return root ;
    }