[Swift] Binary Tree Preorder Traversal

Given the root of a binary tree, return the preorder traversal of its nodes’ values.

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

풀러가기

문제 이해

코드

func preorderTraversal(_ root: TreeNode?) -> [Int] {
  var result: [Int] = []
  preorderTraversal(root, &result)
  return result
}

func preorderTraversal(_ root: TreeNode?, _ arr: inout [Int]) {
  guard let root = root else { return }

  arr.append(root.val)
  preorderTraversal(root.left, &arr)
  preorderTraversal(root.right, &arr)
}

풀이

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