Invert Binary Tree

func invertTree(root *treenode) *treenode {
    var dfs func(root *treenode)
    dfs = func(node *treenode) {
        if node == nil {
            return
        }

        dfs(node.left)
        dfs(node.right)
        node.left, node.right = node.right, node.left
    }

    dfs(root)
    return root
}