type
    BinarySearchTree = ref object
        Value: int
        LeftNode, RightNode: BinarySearchTree

proc isValidBinarySearchTree(tree : BinarySearchTree, lowerBound : int, upperBound : int) : bool =
    if isNil(tree):
        return true
    elif tree.Value > upperBound or tree.Value < lowerBound:
        return false
    elif not isValidBinarySearchTree(tree.LeftNode, lowerBound, tree.Value):
        return false
    elif not isValidBinarySearchTree(tree.RightNode, tree.Value, upperBound):
        return false
    else:
        return true

let example1 =
    BinarySearchTree(Value: 5,
        LeftNode: BinarySearchTree(Value: 1),
        RightNode: BinarySearchTree(Value: 4,
            LeftNode: BinarySearchTree(Value: 3),
            RightNode: BinarySearchTree(Value: 6)))

let example2 =
    BinarySearchTree(Value: 5,
        LeftNode: BinarySearchTree(Value: 1),
        RightNode: BinarySearchTree(Value: 6,
            LeftNode: BinarySearchTree(Value: 4),
            RightNode: BinarySearchTree(Value: 7)))

let example3 =
    BinarySearchTree(Value: 5,
        LeftNode: BinarySearchTree(Value: 1),
        RightNode: BinarySearchTree(Value: 6,
            LeftNode: BinarySearchTree(Value: 7),
            RightNode: BinarySearchTree(Value: 8)))

let example4 =
    BinarySearchTree(Value: 5,
        LeftNode: BinarySearchTree(Value: 1),
        RightNode: BinarySearchTree(Value: 7,
            LeftNode: BinarySearchTree(Value: 6),
            RightNode: BinarySearchTree(Value: 8)))

echo "Example1: ", isValidBinarySearchTree(example1, low(int), high(int))
echo "Example2: ", isValidBinarySearchTree(example2, low(int), high(int))
echo "Example3: ", isValidBinarySearchTree(example3, low(int), high(int))
echo "Example4: ", isValidBinarySearchTree(example4, low(int), high(int))