Write code to partition a linked list around a value x, such that all nodes less than x come before all nodes greater than or equal to x. If x is contained within the list, the values of x only need to be after the elements less than x (see below). The partition element x can appear anywhere in the "right partition"; it does not need to appear between the left and right partitions.

Example:

Input: head = 3->5->8->5->10->2->1, x = 5

Output: 3->1->2->10->5->5->8