/usr/include/stxxl/bits/algo/ is in libstxxl-dev 1.3.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/stxxl/bits/algo/adaptor.h
/usr/include/stxxl/bits/algo/async_schedule.h
/usr/include/stxxl/bits/algo/inmemsort.h
/usr/include/stxxl/bits/algo/intksort.h
/usr/include/stxxl/bits/algo/ksort.h
/usr/include/stxxl/bits/algo/losertree.h
/usr/include/stxxl/bits/algo/random_shuffle.h
/usr/include/stxxl/bits/algo/run_cursor.h
/usr/include/stxxl/bits/algo/scan.h
/usr/include/stxxl/bits/algo/sort.h
/usr/include/stxxl/bits/algo/sort_base.h
/usr/include/stxxl/bits/algo/sort_helper.h
/usr/include/stxxl/bits/algo/stable_ksort.h