一般树的 BFS 层次遍历

func simpleLevelOrder(root *TreeNode) []string {
	if root == nil {
		return nil
	}
	var result []string
	currLever := []*TreeNode{root}
	for len(currLever) > 0 {
		var nextLever []*TreeNode
		for _, v := range currLever {
			result = append(result, v.Val)
			for _, c := range v.Children {
				nextLever = append(nextLever, c)
			}
		}
		currLever = nextLever
	}
	return result
}

// 迭代版。借助一个queue实现，或者用数组也可以
func levelOrder(root *TreeNode) [][]string {
	if root == nil {
		return nil
	}
	var result [][]string
	queue := list.New()
	queue.PushBack(root)
	for queue.Len() > 0 {
		var values []string
		total := queue.Len()
		for i := 0; i < total; i++ {
			node := queue.Remove(queue.Front()).(*TreeNode)
			values = append(values, node.Val)
			if len(node.Children) == 0 {
				continue
			}
			for _, v := range node.Children {
				queue.PushBack(v)
			}
		}
		result = append(result, values)
	}
	return result
}