Preorder Traversal BST

PreorderValidBST.java

public static boolean validBST(int[] arr){
    int lowerbound = Integer.MIN_VALUE;
    Stack<Integer> stack = new Stack<Integer>();
    
    for(int i : arr){
        if(i < lowerbound) return false;
        while(!stack.isEmpty() && i > stack.peek()){
            lowerbound = stack.pop();
        }
        stack.push(i);
    }
    return true;
}