package q114

type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}

func fl(node *TreeNode) (head, tail *TreeNode) {
	if node == nil {
		return nil, nil
	}
	head, tail = node, node

	lhead, ltail := fl(node.Left)
	rhead, rtail := fl(node.Right)

	if lhead != nil {
		node.Left = nil
		node.Right = lhead
		tail = ltail
	}

	if rhead != nil {
		tail.Right = rhead
		tail = rtail
	}
	return
}

func flatten(root *TreeNode) {
	fl(root)
}

var _ = flatten