Given an array of unique integers preorder, return true if it is the correct preorder traversal sequence of a binary search tree. Example 1: Input: preorder = [5,2,1,3,6] Output: true Example 2: Input: preorder = [5,2,6,1,3] Output: false Constraints: 1 <= preorder.length <= 104 1 <= preorder[i] <= 104 All the elements of preorder are unique. Follow up: Could you do it using only constant space complexity?