4.4 Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D,you'll have D linked lists).

4.4 isValidBST

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

public boolean isValidBST(TreeNode root) {
    if (root == null) {
        return true;
    }
        
    TreeNode[] pre = new TreeNode[1];
    pre[0] = null;
    return helper(root, pre);
}
    
private boolean helper(TreeNode root, TreeNode[] pre) {
    if (root == null) {
        return true;
    }
        
    if (!helper(root.left, pre)) {
        return false;
    }
        
    if (pre[0] != null && pre[0].val >= root.val) {
        return false;
    }
        
    pre[0] = root;
        
    if (!helper(root.right, pre)) {
        return false;
    }
        
    return true;
}