Method checking whether a binary tree is also a BST

BSTValidator.m

#import <Foundation/Foundation.h>

@interface Node: NSObject

@property (nonatomic, readonly, assign) int key;
@property (nonatomic, readonly, strong) id value;
@property (nonatomic, strong) Node *left;
@property (nonatomic, strong) Node *right;

+ (Node*) nodeWithKey: (int) key value: (id) value;
+ (BOOL) isBinarySearchTree: (Node*) tree;
- (instancetype) initWithKey: (int) key value: (id) value;

@end

@implementation Node 

+ (Node*) nodeWithKey:(int)key value:(id)value {
	return [[Node alloc] initWithKey: key value: value];
}

+ (BOOL) isBinarySearchTree: (Node*) tree
{
	if(tree == nil) return NO;
	return [self check: tree leftBoundary: INT_MIN rightBoundary: INT_MAX];
}

+ (BOOL) check: (Node*) node leftBoundary: (int) left rightBoundary: (int) right
{
	if(node == nil) return YES;
	BOOL leftSubtree = [self check: node.left leftBoundary: left rightBoundary: node.key];
	BOOL rightSubtree = [self check: node.right leftBoundary: node.key rightBoundary: right];
	return leftSubtree && rightSubtree && (node.key >= left) && (node.key <= right);
}

- (instancetype) initWithKey:(int)key value:(id)value {
	self = [super init];
	if(self) {
		_key = key;
		_value = value;	
	}
	return self;
}

@end



int main(int argc, char *argv[]) {
	@autoreleasepool {
		Node *objectiveC = [Node nodeWithKey: 10 value: @"objectiveC"];
		Node *go = [Node nodeWithKey: 2 value: @"go"];
		Node *python = [Node nodeWithKey: 6 value: @"python"];
		Node *cpp= [Node nodeWithKey: 3 value: @"cpp"];
		Node *java = [Node nodeWithKey: 4 value: @"java"];
		
		python.left = cpp;
		python.right = objectiveC;
		cpp.left = go;
		cpp.right = java;
		
		NSLog(@"Tree is BST: %i", [Node isBinarySearchTree: python]);
	}
}