排序链表

题目链接：排序链表

/**
 * 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* sortList(ListNode* head) {
      if(head == NULL || head->next == NULL){
          return head;
      }

      return mergeSort(head);
    }

    //归并排序
    ListNode* mergeSort(ListNode* pBegin){
        //递归的出口（终止条件）
        if(pBegin == NULL || pBegin->next == NULL){
            return pBegin;
        }
        //首先找到中间结点
        ListNode* pMid = findMidPointer(pBegin);
        //对前半部分进行归并排序
        ListNode* la = mergeSort(pBegin);
        //对后半部分进行归并排序
        ListNode* lb = mergeSort(pMid);
        //合并前后两个部分的链表
        return mergeList(la,lb);
    }
    ListNode* findMidPointer(ListNode *start){
        //快慢指针,相同七点，快指针走两步，慢指针走一步，那么最后慢指针所指向的就是中间结点
        ListNode *fast,*slow,*pre;
        fast = slow = start;
        pre = slow;
        while(fast && fast->next){
            pre = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        pre->next = NULL;
        return slow;
    }
    ListNode* mergeList(ListNode* La,ListNode* Lb){
        ListNode *p,*q,*r;
        p = La;
        q = Lb;
        ListNode *head = new ListNode(0);
        head->next = NULL;
        r = head;
        while(p&&q){
            if(p->val < q->val){
                r->next = p;
                r = p;
                p = p->next;
            }else{
                r->next = q;
                r = q;
                q = q->next;
            }

        }
        while(p){
            r->next = p;
            r = p;
            p = p->next;
        }
        while(q){
            r->next = q;
            r = q;
            q = q->next;
        }
        r->next = NULL;
        return head->next;
    }
};