Links

32.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)
著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。

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
}
3