Determines whether a node is part of a valid binary search tree

IsValidBST

// isValidBST() should return whether or not a given node is a valid binary search tree
//
// Assumes the following object:
// @interface Node
// @property (nonatomic) Node *left;
// @property (nonatomic) Node *right;
// @property (nonatomic) NSNumber *value;
// @end

- (BOOL)isValidBST(Node *root) {
  NSMutableArray *inOrderArrayValues = [NSMutableArray array];
  BOOL isValid = YES;

  if(_checkValidBST(inOrderArrayValues, root)) {

    // Check the final array values
    // If the array is empty, there were no nodes, so return NO.
    // Otherwise, iterate over the array and verify that the values are monotonically increasing

    if([inOrderArrayValues count] == 0) {
      isValid = NO;
    } else {
      NSInteger lastVal = [inOrderArrayValues objectAtIndex:0];
      for(NSNumber *num in inOrderArrayValues) {
        if([num integerValue] >= lastVal) {
          lastVal = [num integerValue]
        } else {
          isValid = NO;
          break;
        }
      }
    }
  } else {
    isValid = NO;
  }

  return isValid;
}

- (BOOL)_checkValidBST(NSMutableArray *arrValues, Node *root) {

  if(root == null) {
    return YES;
  } else {
    
    // First check the left subtree. If valid, verify that the previous element is less than or equal to the current value.
    BOOL checkSubtree = _checkValidBST(arrValues, root.left);
    if(checkSubtree) {

      // Check the previous node in the arrValues array, if not empty
      // If the previous node's value is greater than our current value, this is not a valid BST
      if(_isLastValueLessThanNode(arrValues, root)) {
        [arrValues addObject:root.value];
        return _checkValidBST(arrValues, root.right);
      } else {
        return NO;
      }
    } else {
      return checkSubtree;
    }
  }
}

- (BOOL)_isLastValueLessThanNode(NSMutableArray *arrValues, Node *root) {
  NSInteger totalElements = [arrValues count];
  if(totalElements > 0 && [[arrValues objectAtIndex:(totalElements-1)] integerValue] > [root.value integerValue]) {
    return NO;
  } else {
    return YES;
  }
}