Implement the radixsort algorithm to base 2¹⁶ for sorting 32-bit unsigned integers given in a linked list. The structure of the linked list is

struct listnode { struct listnode * next; unsigned int key;};

You write a function

struct listnode * sort(struct listnode * a);

which returns the listnodes sorted in increasing order. You do not allocate new nodes (or lose track of the given ones).

The programming language is C or C++;