Identifying the prefix sum pattern

The prefix sum technique can only be used to solve some specific problems. These are generally easy or medium problems involving arrays or strings, where we must map the aggregate value of a function over all prefixes of the sequence to the indices where the prefix ends. The aggregate values mapped to indices are then used along with other problem contexts to solve the problem partially or completely. If the problem statement or its solution follows the generic template below, it can be solved by applying the counting technique.

Example