找出一个数组的最大值

 NSArray *sortDataArray = [convertArr sortedArrayUsingComparator:^NSComparisonResult(id  _Nonnull obj1, id  _Nonnull obj2) {
        return [obj2 compare:obj1]; //降序
    }];

int max = [[convertArr valueForKeyPath:@"@max.intValue"] intValue];