** 用一个 ltoR 标识标记本行是否 left to right，每次遍历完一行就翻转标志位 package main import . "nc_tools" func Print(pRoot *TreeNode) (ans [][]int) { if pRoot==nil{ return } queue := []*TreeNode{pRoot} lToR := true for len(queue) != 0 { size := len(queue) layer := make([]int, size) for i := 0; i < size; i++ { node := queue[0] queue = queue[1:] if lToR { layer[i] = node.Val } else { layer[size-1-i] = node.Val } if node.Left != nil { queue = append(queue, node.Left) } if node.Right != nil { queue = append(queue, node.Right) } } ans = append(ans, layer) lToR = !lToR } return }