這也是新手滿常見的問題， nsdictionary 裡有 name 跟 score 的值，產生一個由低分到高分的 keys 陣列。
這個方法可以自訂一些條件，nsmutabledictionary也同樣適用喔。

NSMutableDictionary *dict = [NSDictionary dictionaryWithObjectsAndKeys:
                          
                          [NSDictionary dictionaryWithObjectsAndKeys:@"mark", @"name", @"100", @"score", nil], @"mark",
                          [NSDictionary dictionaryWithObjectsAndKeys:@"tom", @"name", @"78", @"score", nil], @"tom",
                          [NSDictionary dictionaryWithObjectsAndKeys:@"mary", @"name", @"81", @"score", nil], @"mary",
                          [NSDictionary dictionaryWithObjectsAndKeys:@"sandy", @"name", @"65", @"score", nil], @"sandy",
                          [NSDictionary dictionaryWithObjectsAndKeys:@"sam", @"name", @"98", @"score", nil], @"sam",
                          
                          nil];

NSArray *blockSortedKeys = [dict keysSortedByValueUsingComparator: ^(id obj1, id obj2) {
        
        if ([[obj1 objectForKey:@"score"] intValue] > [[obj2 objectForKey:@"score"] intValue]) {
            
            return (NSComparisonResult)NSOrderedDescending;
        }
        
        if ([[obj1 objectForKey:@"score"] intValue] < [[obj2 objectForKey:@"score"] intValue]) {
            
            return (NSComparisonResult)NSOrderedAscending;
        }
        
        return (NSComparisonResult)NSOrderedSame;

    }];