invert binary tree （recursion）

Question:

Invert a binary tree.
4
/ \
2 7
/ \ /
1 3 6 9
to
4
/ \
7 2
/ \ /
9 6 3 1

Solution:

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