Find the longest string that is a prefix of all strings in a given array. If no common prefix exists, return an empty string.

Find the minimum number of perfect square numbers that sum to a given integer. For example, given n = 12, return 3 because 12 = 4 + 4 + 4.

Reverse a portion of a linked list. You're given the head of the list and start/end positions; reverse only the nodes between those positions.