/// <summary>
/// Determines if a tree is a valid BST or not.
/// </summary>
/// <returns><c>true</c> if the tree is a valid BST; otherwise, <c>false</c>.</returns>
/// <param name="root">Root node of the tree.</param>
public static bool IsValidBST(Node root)
{
try
{
if (root == null)
return(true);
//check if the root node's value is less than or equal to left //child node's biggest value
if (root.Left != null)
{
int leftChildMaxVal = SubNodeMaxValue (root.Left);
if (root.Value < leftChildMaxVal)
return(false);
}
//check if the root node's value is greater than or equal to right //child node's smallest value
if (root.Right != null)
{
int rightChildMinVal = SubNodeMinValue (root.Right);
if (root.Value > rightChildMinVal)
return(false);
}
//Recursively checking the validity
if (!IsValidBST(root.Left) || !IsValidBST(root.Right))
return(false);
return(true);
}
catch (ApplicationException e)
{
Console.WriteLine("Exception: {0}", e.Message);
return(false);
}
}
//To find the smallest value of sub node
static int SubNodeMinValue(Node node)
{
int childMinValue = 0;
int minValue = 0;
if (node.Left == null)
{
childMinValue = node.Value;
}
else
{
childMinValue = SubNodeMinValue (node.Left);
}
if (node.Value < childMinValue)
{
minValue = node.Value;
}
else
{
minValue = childMinValue;
}
return minValue;
}
//To find the biggest value of sub node
static int SubNodeMaxValue(Node node)
{
int childMaxVal = 0;
int maxvalue = 0;
if (node.Right == null)
{
childMaxVal = node.Value;
}
else
{
childMaxVal = SubNodeMaxValue (node.Right);
}
if (node.Value > childMaxVal)
{
maxvalue = node.Value;
}
else
{
maxvalue = childMaxVal;
}
return maxvalue;
}
public Node(int value, Node left, Node right)
{
Value = value;
Left = left;
Right = right;
}