Partition

Write code to partition a list around a value x, such that all elements less than x come before all elements greater than or equal to x.

The element x can appear anywhere in the "right partition".

Example:

Input:  [3, 5, 8, 5, 10, 2, 1], x = 5
Output: [3, 2, 1, 5, 8, 5, 10]  (any valid partition is acceptable)