280 字
1 分钟
III.从上到下打印二叉树III
题目
请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，第三行再按照从左到右的顺序打印，其他行以此类推。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3 / \ 9 20 / \ 15 7返回其层次遍历结果：
[ [3], [20,9], [15,7] ]提示：
节点总数 <= 1000
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof
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题解
func levelOrder(_ root: TreeNode?) -> [[Int]] { guard let r = root else { return [] }
var queue: [TreeNode] = [] var queueB: [TreeNode] = [] var toBePrint = 0 var nextLevelCount = 0 var result: [[Int]] = [] var flag = true
queue.append(r) toBePrint += 1
var temp = Array(repeating: 0, count: queue.count)
while queue.isEmpty == false { let first = queue.removeFirst() temp[flag ? (temp.count - toBePrint) : (toBePrint - 1)] = first.val toBePrint -= 1
if let left = first.left { queueB.append(left) nextLevelCount += 1 }
if let right = first.right { queueB.append(right) nextLevelCount += 1 }
if toBePrint == 0 { result.append(temp) queue = queueB temp = Array(repeating: 0, count: nextLevelCount)
toBePrint = nextLevelCount nextLevelCount = 0 queueB = [] flag.toggle() } }
return result}