Create Rotate List

Given a list, rotate the list to the right by k places, where k is non-negative.

For example:
Given 1->2->3->4->5->NULL and k = 2,
return 4->5->1->2->3->NULL.(当到达list.length的时候，cur的情况容易考虑，pre的也要考虑的，也就是说head本身有可能是newhead)