Given a singly linked list L: L0→_L_1→…→_L_n-1→_L_n,reorder it to: L0→_L_n→_L_1→_L_n-1→_L_2→_L_n-2→…
You may not modify the values in the list's nodes, only nodes itself may be changed.
Example 1:
Given 1->2->3->4, reorder it to 1->4->2->3.
Example 2:
Given 1->2->3->4->5, reorder it to 1->5->2->4->3.