【Leetcode】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 must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

快慢指针找到链表中点，将链表分为两段，翻转后半段，再合并两个子链表。

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     void reorderList(ListNode *head) {
         if (head == nullptr || head->next == nullptr || head->next->next == nullptr) {
             return;
         }
         ListNode *fast = head, *slow = head;
         while (fast->next && fast->next->next) {
             fast = fast->next->next;
             slow = slow->next;
         }
         fast = slow->next;
         slow->next = nullptr;
         fast = reverseList(fast);
         head = mergeList(head, fast);
     }
 private:
     ListNode * reverseList(ListNode *head) {
         ListNode dummy(-);
         ListNode *p = &dummy;
         while (head) {
             ListNode *temp = head->next;
             head->next = p->next;
             p->next = head;
             head = temp;
         }
         return dummy.next;
     }

     ListNode *mergeList(ListNode *l1, ListNode *l2) {
         ListNode dummy(-);
         ListNode *p = &dummy;
         while (l1) {
             p->next = l1;
             l1 = l1->next;
             p = p->next;
             if (l2) {
                 p->next = l2;
                 l2 = l2->next;
                 p = p->next;
             }
         }
         return dummy.next;
     }
 };