題目連結: https://leetcode.com/problems/binary-tree-preorder-traversal/
Iterative version:
func preorderTraversal(_ root: TreeNode?) -> [Int] {
guard let root = root else { return [] }
var stack: [TreeNode] = [root]
var arr: [Int] = []
while !stack.isEmpty {
let last = stack.removeLast()
arr.append(last.val)
if let right = last.right {
stack.append(right)
}
if let left = last.left {
stack.append(left)
}
}
return arr
}Recursive version 1:
func preorderTraversal(_ root: TreeNode?) -> [Int] {
guard let root = root else {
return []
}
var arr = [Int]()
helper(root, arr: &arr)
return arr
}
func helper(_ root: TreeNode?, arr: inout [Int]) {
guard let root = root else { return }
arr.append(root.val)
helper(root.left, arr: &arr)
helper(root.right, arr: &arr)
}
Recursive version 2:
func preorderTraversal(_ root: TreeNode?) -> [Int] {
guard let root = root else { return [] }
return [root.val] + preorderTraversal(root.left) + preorderTraversal(root.right)
}