归并排序

递归实现

void merge(int a[],int l1,int r1,int l2,int r2)  //将两个区间合并
{
	int i=l1,j=l2;
	int temp[maxn],index=0;
	while(i<=r1&&j<=r2)
	{
		if(a[i]<=a[j]) temp[index++]=a[i++];
		else temp[index++]=a[j++];
	}
	while(i<=r1) temp[index++]=a[i++];
	while(j<=r2) temp[index++]=a[j++];
	for(int i=0;i<index;i++) a[l1+i]=temp[i];
}
void mergesort(int a[],int left,int right)    //对当前区间进行归并排序
{
	if(left<right)
	{
		int mid=(left+right)/2;
		mergesort(a,left,mid);
		mergesort(a,mid,right);
		merge(a,left,mid,mid+1,right);
	}
}