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;
}