数据结构--链表--单链表归并排序mergesort

思路：

1.将链表的中点找到，对其切分成2条

2.继续步骤1，切成4条，8条。。。,直至每段链表只有1个元素

3.归并操作，对两两链表进行合并排序，并返回回并后的链表的头结点，依次向上递归回去

C++代码实现

链表头文件链接：https://github.com/hitskyer/course/tree/master/dataAlgorithm/chenmingming/linkedList/homework

//归并排序
// Created by mingm on 2019/3/23.
//
#include <iostream>
#include <time.h>
#include <cstdlib>
#include "./homework/singleList.cpp"
using namespace std;ListNode GetMidNode(ListNode s)   //快慢指针法,获取中间节点
{ListNode fast, slow;fast = s->pNext;//让快指针早点到达末位，慢指针指向中点或者(偶数个长度时)中点前一个位置slow = s;while(fast && fast->pNext){fast = fast->pNext->pNext;slow = slow->pNext;}return slow;
}
ListNode mergeList(ListNode L, ListNode R)  //归并函数
{if(L == NULL)   //如果一边为空，则返回另一边的头结点return R;if(R == NULL)return L;ListNode tempL = L, tempR = R;  //把左右表头存储起来ListNode temp = new SNode, emptyHead = temp; //利用一个空表头哨兵tempwhile(tempL && tempR)       //左右链表均不为空的话，进行数据比较{if(tempL->data < tempR->data){temp->pNext = tempL;temp = temp->pNext;tempL = tempL->pNext;}else{temp->pNext = tempR;temp = temp->pNext;tempR = tempR->pNext;}}if(tempL)                   //如果左边还有剩余的节点，把其接入链表末尾temp->pNext = tempL;if(tempR)                   //如果右边还有剩余的节点，把其接入链表末尾temp->pNext = tempR;temp = emptyHead->pNext;     //实际链表数据节点表头delete emptyHead;           //释放new出来的哨兵emptyHead = NULL;return temp;                //返回链表数据头结点
}
ListNode divList(ListNode Lhead)
{if(Lhead == NULL || Lhead->pNext == NULL)   //链表长度为0或者1，不用排序return Lhead;ListNode Mid = GetMidNode(Lhead);   //获取链表中间节点(如果长度为2，则Mid是第一个节点)ListNode Rhead = Mid->pNext;    //右边链表表头地址Mid->pNext = NULL;              //断开左右链表ListNode L = divList(Lhead);    //继续对左右两条链表进行划分ListNode R = divList(Rhead);return mergeList(L,R);          //返回merge后的链表的表头
}
ListNode mergeSort(ListNode head)   //归并排序入口，将头结点地址传入
{if(head == NULL || head->pNext == NULL) //链表长度为0或者1，不用排序return head;elsedivList(head);  //长度大于1，则对其进行划分将链表切片
}int main()
{srand((unsigned)time(NULL));    //用时间随机数种子size_t len = 10;       //测试链表最大长度for(size_t j = 0; j < len; ++j){SingleList intList;for(size_t i = 0; i < j; ++i){intList.AddTail(rand()%100);    //添加随机数到链表}cout << "before merge sort: " << endl;intList.PrintList();    //排序前链表打印intList.SetHeadNode(mergeSort(intList.GetHeadNode()));  //把排序后的链表的头结点设置成链表的头结点cout << "after merge sort: " << endl;intList.PrintList();    //排序后链表打印}return 0;
}