[LC] 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

class Solution {
    public boolean verifyPreorder(int[] preorder) {
        if (preorder == null) {
            return false;
        }
        // keep a decresing stack
        LinkedList<Integer> stack = new LinkedList<>();
        int min = Integer.MIN_VALUE;
        for (int node: preorder) {
            if (node < min) {
                return false;
            }
            // get into right subtree
            while (!stack.isEmpty() && node > stack.peekFirst()) {
                min = stack.pollFirst();
            }
            stack.offerFirst(node);
        }
        return true;
    }
}