Simple Task with Binary Tree written in Objective-C.

BinaryTree.h

/**
 * TreeNode:
 *    -val: NSNumber
 *    -left: TreeNode
 *    -right: TreeNode
 */
 
// Task: Build a Balanced Binary Search Tree from an Ordered Array.
- (TreeNode *)buildMinBSTFromArray:(NSArray *)array;

// Task: Level Traverse Binary Tree.
- (void)printBinaryTreeAtNode:(TreeNode *)root;

// Task: Check if a Binary Search Tree is valid.
- (BOOL)checkBST:(TreeNode *)root;

BinaryTree.m

- (TreeNode *)buildMinBSTFromArray:(NSArray *)array
{
    return [self buildMinBSTForArray:array from:0 to:array.count-1];
}

- (TreeNode *)buildMinBSTForArray:(NSArray *)array from:(NSInteger)left to:(NSInteger)right
{
    if (left > right) return nil;
    if (left == right) return [[TreeNode alloc] initWithValue:array[left]];
    NSInteger mid = (left + right) / 2;
    TreeNode *node = [[TreeNode alloc] initWithValue:array[mid]];
    node.left = [self buildMinBSTForArray:array from:left to:mid-1];
    node.right = [self buildMinBSTForArray:array from:mid+1 to:right];
    return node;
}

// Using NSMutableArray as a Queue to maintain the order of TreeNodes.
- (void)printBinaryTreeAtNode:(TreeNode *)root
{
    NSMutableArray *q = [NSMutableArray array];
    [q addObject:root];
    NSInteger cur = 1;
    NSInteger nxt = 0;
    while(q.count > 0) {
        TreeNode *node = q[0];
        [q removeObjectAtIndex:0];
        printf("%d ", [node.val intValue]);
        cur--;
        if (node.left != nil) {
            [q addObject:node.left];
            nxt++;
        }
        if (node.right != nil) {
            [q addObject:node.right];
            nxt++;
        }
        // All current level's nodes have been visited
        if(cur == 0) {
            cur = nxt;
            nxt = 0;
            printf("\n");
        }
    }
}

- (BOOL)checkBST:(TreeNode *)root
{
    return [self checkBST:root withMax:NSIntegerMax andMin:NSIntegerMin];
}

- (BOOL)checkBST:(TreeNode *)root withMax:(NSInteger)max andMin:(NSInteger)min
{
    if(root == nil) return YES;
    NSInteger val = [root.val integerValue];
    if (val <= min || val >= max) return NO;
    if (![self checkBST:root.left withMax:val andMin:min]) return NO;
    if (![self checkBST:root.right withMax:max andMin:val]) return NO;
    return true;
}