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# 32.I.从上到下打印二叉树

## 题目

从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。

例如:

给定二叉树: \[3,9,20,null,null,15,7],

```
     3
    / \
   9  20
     /  \
    15   7
```

返回：

```
[3,9,20,15,7]
```

提示：

节点总数 <= 1000

来源：[力扣（LeetCode）](https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof)

## 题解

```swift
func levelOrder(_ root: TreeNode?) -> [Int] {
    guard let r = root else { return [] }
    var result: [Int] = []
    var queue: [TreeNode] = [r]

    while queue.isEmpty == false {
        let first = queue[0]
        result.append(first.val)

        if let left = first.left {
            queue.append(left)
        }

        if let right = first.right {
            queue.append(right)
        }

        queue.removeFirst()
    }

    return result
}
```

![1](/files/-MiWFVVfrUkcUt07CeWe)