255. Verify Preorder Sequence in Binary Search Tree

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.

Consider the following binary search tree: 

     5
    / \
   2   6
  / \
 1   3

Example 1:

Input:

 [5,2,6,1,3]
Output:

 false

Example 2:

Input:

 [5,2,1,3,6]
Output:

 true

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