collect.go

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

func order1(root *TreeNode) [][]int {
	r := [][]int{}

	collect := func(nodes []*TreeNode) ([]int, []*TreeNode) {
		level := []int{}
		next := []*TreeNode{}

		for _, n := range nodes {
			if n != nil {
				level = append(level, n.Val)
				if n.Left != nil {
					next = append(next, n.Left)
				}
				if n.Right != nil {
					next = append(next, n.Right)
				}
			}
		}
		return level, next
	}

	func() {
		level := []int{}
		next := []*TreeNode{root}
		for {
			level, next = collect(next)
			if len(level) == 0 {
				return
			}
			r = append(r, level)
		}
	}()
  
	return r
}