归并排序 （分治排序）acwing787

#include<iostream>

using namespace std;

const int N=1e5+10;
int a[N],tmp[N];

void merge_sort(int q[],int l,int r)
{
    
    
	if(l>=r) return;//结束条件是  >=  !!!!
	int mid=l+r>>1;//如果这里不加一与下面一行相对应
	merge_sort(q,l,mid),merge_sort(q,mid+1,r);
	
	int k=0,i=l,j=mid+1;//注意i和r的赋值
	while(i<=mid&&j<=r)
		if(q[i]<=q[j]) tmp[k++]=q[i++];
		else tmp[k++]=q[j++];
	while(i<=mid) tmp[k++]=q[i++];
	while(j<=r) tmp[k++]=q[j++];
	
	for(i=l,j=0;i<=r;j++,i++) q[i]=tmp[j];//注意结束条件是  <=r
} 

int main()
{
    
    
	int n;
	cin>>n;
	for(int i=0;i<n;i++) scanf("%d",&a[i]);
	merge_sort(a,0,n-1);
	for(int i=0;i<n;i++) printf("%d ",a[i]); 
	return 0;
}

void merge_sort(int q[],int l,int r)
{
    
    
	if(l>=r) return;
	int mid=l+r>>1;
	merge_sort(q,l,mid),merge_sort(q,mid+1,r);
	
	int k=0,i=l,j=mid+1;
	while(i<=mid&&j<=r)
		if(q[i]<=q[j]) tmp[k++]=q[i++];
		else tmp[k++]=q[j++];
	while(i<=mid) tmp[k++]=q[i++];
	while(j<=r) tmp[k++]=q[j++];
	
	for(i=l,j=0;i<=r;j++,i++) q[i]=tmp[j];
}