143. Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        if (head == null || head.next == null) return;
        ListNode slow = head, fast = head, prev = null;
        while (fast != null && fast.next != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        prev.next = null; // cut at middle
        slow = reverse(slow);
        // merge two lists
        ListNode curr = head;
        while (curr.next != null) {
            ListNode tmp = curr.next;
            curr.next = slow;
            slow = slow.next;
            curr.next.next = tmp;
            curr = tmp;
        }
        curr.next = slow;
    }
    public ListNode reverse(ListNode head){
        ListNode prev = null;
        ListNode cur = head;
        while(cur != null){
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }
        return prev;
    }
}

可以找到中间节点，断开，把后半截单链表reverse一下，再合并两个单链表。