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32.II.从上到下打印二叉树II

题目

从上到下按层打印二叉树,同一层的节点按从左到右的顺序打印,每一层打印到一行。
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],
[9,20],
[15,7]
]
提示:
节点总数 <= 1000
来源:力扣(LeetCode)

题解

func levelOrder(_ root: TreeNode?) -> [[Int]] {
var queue: [TreeNode] = []
var toBePrint = 0
var nextLevelCount = 0
var result: [[Int]] = []
var temp: [Int] = []
if let r = root {
queue.append(r)
toBePrint += 1
}
while queue.isEmpty == false {
if let first = queue.first {
temp.append(first.val)
queue.removeFirst()
toBePrint -= 1
if let left = first.left {
queue.append(left)
nextLevelCount += 1
}
if let right = first.right {
queue.append(right)
nextLevelCount += 1
}
if toBePrint == 0 {
result.append(temp)
temp = []
toBePrint = nextLevelCount
nextLevelCount = 0
}
}
}
if temp.isEmpty == false {
result.append(temp)
}
return result
}
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