https://leetcode.com/problems/binary-tree-preorder-traversal/ Swift solution

BinaryTreePreorderTraversal.swift

func preorderTraversal(root:TreeNode) -> [Int]{
    var arr = [Int]()
   
    var nodes = [TreeNode]()
    nodes.append(root)
    while !nodes.isEmpty{
        let current = removeLast(&nodes)
        arr.append(current.val)
        if let right = current.right{
            nodes.append(right)
        }
        if let left = current.left{
            nodes.append(left)
        }
    }
    return arr
}