Given an array of numbers, verify whether it is the correct preorder traversal sequence of a binary search tree.

You may assume each number in the sequence is unique.

Follow up:
Could you do it using only constant space complexity?

給定一個數字數組，驗證它是否是二進制搜索樹的正確的預排序遍歷序列。

您可以假定序列中的每個數字都是唯一的。

進階：

你能只用恒定的空間復雜性來做嗎？

 1 class Solution {
 2     func verifyPreorder(_ preorder:[Int]) -> Bool{
 3         var preorder = preorder
 4         var low:Int = Int.min
 5         var i:Int = -1
 6         for a in preorder
 7         {
 8             if a < low
 9             {
10                 return false
 

11             }
12             while(i >= 0 && a > preorder[i])
13             {
14                  low = preorder[i]
15                 i -= 1
16             }
17             i += 1
18             preorder[i] = a
19         }
20         return true
21     }
22 }

[Swift]LeetCode255.驗證二叉搜索樹的先序序列 $ Verify Preorder Sequence in Binary Search Tree