LeetCode OJ

题目：

　　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}.

解题思路：

　　1、用快慢指针找到链表的中点，并将链表分割成两部分

　　2、翻转后面的部分

　　3、将前后两部分拼接

/**
 * 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 == NULL) return;

        //find the last part
        ListNode *mid = head, *last = head;
        while (last != NULL) {
            last = last->next;
            if (last == NULL) break;
            last = last->next;

            mid = mid->next;
        }
        ListNode *right_head = mid->next;
        mid->next = NULL;

        //翻转 右边部分
        ListNode *pre = NULL, *suf = right_head;
        while (suf != NULL) {
            ListNode * tmp = suf->next;
            if (tmp == NULL) {
                right_head = suf;
            }
            suf->next = pre;
            pre = suf;
            suf = tmp;
        }

        //拼接
        ListNode *cur = head;
        while (cur != NULL && right_head != NULL) {
            ListNode *tmp = right_head->next;
            right_head->next = cur->next;
            cur->next = right_head;
            right_head = tmp;
            cur = cur->next->next;
        }
    }
};