every entry of val has an id, we will arrange val such that the result matches the given id array */.

SHA256 hash for file content.

index of the first element equaling to v, otherwise return -1.

use binary search.

resort an sorted array after one entry has modified */.

根据字符串推断程序语言.

Whether a is start with b */.

dependon: whether i dependon j.

comparable.