獲取NSArray中元素的和
NSNumber *sum = [countArray valueForKeyPath:@"@sum.floatValue"];
獲取NSArray中元素的最大值
NSInteger max = [[countArray valueForKeyPath:@"@max.intValue"] integerValue];
NSNumber *sum = [countArray valueForKeyPath:@"@sum.floatValue"];
NSInteger max = [[countArray valueForKeyPath:@"@max.intValue"] integerValue];