Leetcode Reorder List

实现链表如下所示：

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

一开始想到一个n方的，就是每次找最后一个回调到相应位置，然后倒数第二个的next置为NULL，依次类推。果然超时。

/**
 * 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 || !head->next) return ;
        ListNode *cur, *last, *next_cur, *pre_last;
        last = head -> next;
        pre_last = head;
        while(last -> next)
        {
            pre_last = last;
            last = last -> next;
        }
        cur = head;
        next_cur = head -> next;

        while(cur -> next || cur -> next != last)
        {
            cur -> next = last;
            pre_last -> next = NULL;
            cur = next_cur;
            next_cur = cur -> next;
            last = cur -> next;
            pre_last = cur;
            while(last && last -> next)
            {
                pre_last = last;
                last = last -> next;
            }
        }
        return ;
    }
};

也可以用map<int, ListNode*>来做吧应该，但用了其他空间了。下面是比较直接的没有用多余空间的解法。

分成两半，后面一半反转，然后合并前后两半。

需要注意的是，前面一半的最后一个记得赋值NULL，还有如果是奇数，那么前面一半应该多一个。自己随便举个1234和12345的例子画一下就知道怎么合并怎么分了。

/**
 * 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 || !head -> next) return ;
        ListNode *slow = head, *quick = head -> next;
        while(quick && quick -> next) // 分成两部分
        {
            slow = slow -> next;
            quick = quick -> next;
            if (!quick)
                break;
            quick = quick -> next;
        }
        quick = slow -> next;
        ListNode *last = quick -> next;
        while(last) // 反转后半部
        {
            quick -> next = last -> next;
            last -> next = slow -> next;
            slow -> next = last;
            last = quick -> next;
        }
        quick = slow -> next;
        slow -> next = NULL;

        ListNode *next_cur = head -> next, *next_quick, *cur = head;
        while(cur && quick) // 合并两部分
        {
            cur -> next = quick;
            next_quick = quick -> next;
            quick -> next = next_cur;
            quick = next_quick;
            cur = next_cur;
            if (cur)
                next_cur = cur -> next;
        }
    }   
};