leetcode143 重排链表

思路：

记录一种递归的方法。

实现：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution
{
public:
    ListNode* work(ListNode* head, int len)
    {
        if (len == 1) return head;
        if (len == 2) return head->next;
        ListNode* tail = work(head->next, len - 2);
        ListNode* hn = head->next;
        ListNode* tmp = tail->next->next; 
        head->next = tail->next;
        tail->next->next = hn;
        tail->next = tmp;
        return tail;
    }
    void reorderList(ListNode* head)
    {
        if (!head) return;
        int len = 0;
        ListNode* tmp = head;
        while (tmp) { len++; tmp = tmp->next; }
        work(head, len);
    }
};