归并排序，递归与非递归

递归：

void merge_array(int a[],int low,int mid,int end){
    int* temp =  new int[end-low+1];
    int index_pre = low;
    int index_post = mid+1;
    int i=0;
    while(index_pre<=mid&&index_post<=end){
        if(a[index_pre]<a[index_post])
            temp[i++] = a[index_pre++];
        else
            temp[i++] = a[index_post++];
    }
    if(index_pre<=mid)while(index_pre<=mid)
            temp[i++] = a[index_pre++];
    if(index_post<=end)while(index_post<=end)
            temp[i++] = a[index_post++];
    for(i=0;i<end-low+1;i++)
        a[low+i] = temp[i];
    delete[] temp;
}

void merge_sort(int a[],int low, int high){
    if(low<high){
        int mid = (low+high)/2;
        merge_sort(a,low,mid);
        merge_sort(a,mid+1,high);
        merge_array(a,low,mid,high);
    }
}                

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)。

void mergeArray2(int a[],int b[], int low,int mid,int high){
    int i=low,j=mid+1,k=i;
    while(i<=mid&&j<=high){
        if(b[i]<b[j]){
            a[k++]=b[i++];
        }else{
            a[k++]=b[j++];
        }
    }
    while(j<=high)
        a[k++]=b[j++];
    while(i<=mid)
        a[k++]=b[i++];
}
void mergepass(int a[],int b[],int s,const int length){
    int i=0;
    while(i<=length-2*s){
        mergeArray2(a,b,i,i+s-1,i+2*s-1);
        i = i+2*s;
    }
    if(i+s<length)mergeArray2(a,b,i,i+s-1,length-1);
    else for(int j=i;j<length;j++)a[j]=b[j];
}
void mergeSort(int temp[],const int length){
    int i=0;
    int s=1;
    int* temp1= new int[length];
    while(s<length){
        mergepass(temp1,temp,s,length);
        s*=2;
        mergepass(temp,temp1,s,length);
        s*=2;
    }
    delete[] temp1;
}