# 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）

链接：<https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof>

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

```swift
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](https://4193904735-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MI8JgbGh3U6X_oedqkm%2Fsync%2F1a7b6702069b4851345c450cd5ccce5f94310fa6.png?generation=1630498067689588\&alt=media)