归并排序

    归并排序的概念和原理我就不介绍了，网上的相关资料一箩筐。在这里，我只想记录一个思路，归并的排序分为三步走：1 分割，2 递归，3 合并。下面我将分别针对数组和链表两种情况的归并排序，写一下程序和思路。关于链表的归并排序，这个题目我在网易有道的面试中经历过。当时蒙住了，因为绝大多数的数据结构的书，以及算法的书介绍归并排序的时候往往都是以数组为例的。

    数组归并排序代码：

 

/*

**归并排序三步走：1 分割子问题；2 递归；3 合并子问题。

*/

#include "stdafx.h"

#include <iostream>

using namespace std;

void mergeArray(int *a, int begin1, int length1, int begin2, int length2)

{

    int length = length1 + length2;

    int begin = begin1;

    int *b = new int[length];

    int pos = 0;

    int cur1 = begin1;

    int cur2 = begin2;

    while((cur1 - begin1 + 1) <= length1 && (cur2 - begin2 + 1) <= length2)

    {

      if(a[cur1] <= a[cur2])

      {

          b[pos] = a[cur1];

          cur1++;

          pos++;

      }

      else

      {

          b[pos] = a[cur2];

          cur2++;

          pos++;

      }

    }

    //前半段的数组已经合并完毕，可以直接将后半段的数组复制到数据b

    if((cur1 - begin1 + 1) > length1)

    {

      while((cur2 - begin2 + 1) <= length2)

      {

          b[pos] = a[cur2];

          cur2++;

          pos++;

      }

    }

    else if((cur2 - begin2 + 1) > length2)

    {

      while((cur1 - begin1 + 1) <= length1)

      {

          b[pos] = a[cur1];

          cur1++;

          pos++;

      }

    }

    //将合并后的数组b复制给数据a

    for(int i = 0; i < length; i++)

    {

      a[begin] = b[i];

      begin++;

    }

}

void mergeSort(int *a, int left, int right)

{

    if(left >= right)

      return;

    int mid = left + (right - left)/2;

    mergeSort(a, left, mid);

    mergeSort(a, mid+1, right);

    int length1 = mid - left + 1;

    int length2 = right -(mid + 1) + 1;

    mergeArray(a, left, length1, mid + 1, length2);

}

int _tmain(int argc, _TCHAR* argv[])

{

    int a[5] = {3, 5, 1, 2, 7};

    //int a[5] = {1,1,2,1,1};

    mergeSort(a, 0, 4);

    for(int i = 0; i < 5; i++)

    {

      cout<<a[i]<<" ";

    }

    getchar();

    return 0;

}

---

---

    对链表进行归并排序的代码如下：

struct node

{

    int data;

    node * next;

};

/*

**对两个有序链表进行归并

*/

node *MergeList(node *head1, node *head2)

{

    node * tmp;

    if(head1 == NULL)

      return head2;

    if(head2 == NULL)

      return head1;

    if(head1->data < head2->data)

    {

      tmp = head1;

      head1 = head1->next;

    }

    else

    {

      tmp = head2;

      head2 = head2->next;

    }

    tmp->next = MergeList(head1, head2);

    return tmp;

}

/*

**归并排序，参数为要排序的链表的头结点，函数返回值为排序后的链表的头结点

*/

node *MergeSort(node *head)

{

    if(head == NULL)

      return 0;

    node * r_head = head;

    node *head1 = head;

    node* head2 = head;

    while(head2->next != NULL && head2->next ->next!= NULL)

    {

      head1 = head1->next;

      head2 = head2->next->next;

    }

    if(head1->next == NULL)/*说明只有一个节点，则返回该节点*/

      return r_head;

    head2 = head1->next;

    head1->next = NULL;

    head1 = head;

    /*函数MergeList是对两个有序链表进行归并，返回值是归并后的链表的头结点*/

    r_head = MergeList(MergeSort(head1), MergeSort(head2));

    return r_head;

}