【Leetcode】Invert Binary Tree

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

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

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