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

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

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