private static bool Helper(LeetCode98TreeNode root, int?lower, int?upper)
{
if (root == null)
{
return(true);
}
if (lower != null && root.val <= lower)
{
return(false);
}
if (upper != null && root.val >= upper)
{
return(false);
}
if (!Helper(root.left, lower, root.val))
{
return(false);
}
if (!Helper(root.right, root.val, upper))
{
return(false);
}
return(true);
}
public static bool IsValidBST(LeetCode98TreeNode root)
{
return(Helper(root, null, null));
}
public LeetCode98TreeNode(int val = 0, LeetCode98TreeNode left = null, LeetCode98TreeNode right = null)
{
this.val = val;
this.left = left;
this.right = right;
}
