归并排序，递归与非递归

递归：

 1 void merge_array(int a[],int low,int mid,int end){
 2     int* temp =  new int[end-low+1];
 3     int index_pre = low;
 4     int index_post = mid+1;
 5     int i=0;
 6     while(index_pre<=mid&&index_post<=end){
 7         if(a[index_pre]<a[index_post])
 8             temp[i++] = a[index_pre++];
 9         else
10             temp[i++] = a[index_post++];
11     }
12     if(index_pre<=mid)while(index_pre<=mid)
13             temp[i++] = a[index_pre++];
14     if(index_post<=end)while(index_post<=end)
15             temp[i++] = a[index_post++];
16     for(i=0;i<end-low+1;i++)
17         a[low+i] = temp[i];
18     delete[] temp;
19 }
20
21 void merge_sort(int a[],int low, int high){
22     if(low<high){
23         int mid = (low+high)/2;
24         merge_sort(a,low,mid);
25         merge_sort(a,mid+1,high);
26         merge_array(a,low,mid,high);
27     }
28 }                

非递归：

即以1,2,4,8...为间隔对数组进行归并，需要用到额外的空间。空间复杂度是O(n)，时间复杂度为O(nlogn)。

 1 void mergeArray2(int a[],int b[], int low,int mid,int high){
 2     int i=low,j=mid+1,k=i;
 3     while(i<=mid&&j<=high){
 4         if(b[i]<b[j]){
 5             a[k++]=b[i++];
 6         }else{
 7             a[k++]=b[j++];
 8         }
 9     }
10     while(j<=high)
11         a[k++]=b[j++];
12     while(i<=mid)
13         a[k++]=b[i++];
14 }
15 void mergepass(int a[],int b[],int s,const int length){
16     int i=0;
17     while(i<=length-2*s){
18         mergeArray2(a,b,i,i+s-1,i+2*s-1);
19         i = i+2*s;
20     }
21     if(i+s<length)mergeArray2(a,b,i,i+s-1,length-1);
22     else for(int j=i;j<length;j++)a[j]=b[j];
23 }
24 void mergeSort(int temp[],const int length){
25     int i=0;
26     int s=1;
27     int* temp1= new int[length];
28     while(s<length){
29         mergepass(temp1,temp,s,length);
30         s*=2;
31         mergepass(temp,temp1,s,length);
32         s*=2;
33     }
34     delete[] temp1;
35 }