import java.util.*;
 
class Main{
	static boolean preorderBST(int input[], int n)
	{
	    Stack<Integer> s = new Stack<Integer>();
	    int root = Integer.MIN_VALUE;
	    for(int i=0;i<n;i++)
	    {
	        // if current node is smaller than the root
	        // the current node lies on the right of root
	        // thus it is impossible for bst
	        if (input[i] < root)
	            return false;
	        // keep removing elements smaller than the root
	        // until you find an element which is greater than the root
	        while(!s.empty() && s.peek()<input[i]){
	            root = s.peek();
	            s.pop();
	        }
	        // if current element is smaller than the root
	        // push it into stack
	        s.push(input[i]);
	    }
	    // if we passed until the end of the array
	    // that means that the given preorder sequence represents a valid BST
	    return true;
	}
 
	public static void main(String[] args)
	{
	    int input[] = {5, 3, 4, 7, 6, 9, 8, 11};
	    int n = input.length;
 
	    System.out.print((preorderBST(input, n)) ? "yes" : "no");
	}
}

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