traverse a json-ish structure in objective-c (and delete certain elements from it)

Traverse.m

+ (BOOL)traverse:(NSObject *)obj comparisonBlock:(BOOL (^)(NSString *, NSObject *))comparisonBlock;
{
    if([obj isKindOfClass:[NSMutableArray class]])
    {
        NSMutableArray *array = (NSMutableArray *)obj;
        
        NSMutableArray *toDelete = [@[] mutableCopy];
        
        for(NSObject *element in array)
        {
            if([self _isContainer:element])
            {
                if([self traverse:element comparisonBlock:comparisonBlock])
                {
                    [toDelete addObject:element];
                }
            }
        }
        
        [array removeObjectsInArray:toDelete];
            
    }
    else if([obj isKindOfClass:[NSMutableDictionary class]])
    {
        NSMutableDictionary *dict = (NSMutableDictionary *)obj;
        NSMutableArray *keysToDelete = [@[] mutableCopy];
        for(NSString *subKey in [dict allKeys])
        {
            if([self _isContainer:dict[subKey]])
            {
                if([self traverse:dict[subKey] comparisonBlock:comparisonBlock])
                {
                    [keysToDelete addObject:subKey];
                }
            }
            else
            {
                // Leaf
                if(comparisonBlock(subKey, dict[subKey]))
                {
                    return YES;
                }
            }
        }
        
        [dict removeObjectsForKeys:keysToDelete];
    }
   
    return NO;
}