143. 重排链表

143. 重排链表

切成前半部分和后半部分，后半部分反转

/**
 * 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:
    void reorderList(ListNode* head) {
        ListNode* walker = head, *runner = head;
        ListNode* pre = nullptr;
        while(runner->next && runner->next->next) 
        {
            walker = walker->next;
            runner = runner->next->next;
        }  
        auto l1 = walker->next;
        walker->next = nullptr;
        while(l1)
        {
            auto next = l1->next;
            l1->next = pre;
            pre = l1;
            l1 = next;
        }
        runner = head;
        while(pre)
        {
            auto next = runner->next;
            runner->next = pre;
            auto t = pre->next;
            pre->next = next;
            pre = t;
            runner = next;
        }
    }
};