经典的归并排序写法

#include<iostream>
using namespace std;
int n;
int a[2500];
//归并排序 
int b[2500];
int merge(int a[],int start,int end,int b[]);
int merge_sort(int a[],int start,int end,int b[]){
	if(start>=end)
	return 0;
	merge_sort(a,start,(start+end)/2,b);
	merge_sort(a,(start+end)/2+1,end,b);
	merge(a,start,end,b);
	return 0;
}
int merge(int a[],int start,int end,int b[]){
	int middle=(start+end)/2;
	int rightindex=middle+1;
	int leftindex=start;
	int index=leftindex;
	while(leftindex<=middle&&rightindex<=end){
		if(a[leftindex]<a[rightindex]){
			b[index++]=a[leftindex++];
		}else{
			b[index++]=a[rightindex++];
		}
	}
	while(leftindex<=middle){
		b[index++]=a[leftindex++]; 
	}
	while(rightindex<=end){
		b[index++]=a[rightindex++];
	}
		for(int i=start;i<=end;i++){
			a[i]=b[i];
		}
		return 0;	
}
int main(){
	
	cin>>n;
	for(int i=0;i<n;i++){
		cin>>a[i];
	}
	int start=0,end=n-1;
	merge_sort(a,start,end,b); 
	for(int i=0;i<n;i++){
		cout<<a[i]<<" ";
	}
	return 0;
}