归并排序

#include<bits/stdc++.h>
using namespace std;
const int N = 1000010;
int n,a[N],b[N];
void merge(int l1,int r1,int l2,int r2){
    int i = l1,j = l2;
    int d = l1;
    while(i<=r1 && j<=r2){
        if(a[i] <= a[j]){   //小到大
            b[d++] = a[i++];
        }
        else{
            b[d++] = a[j++];
        }
    }
    while(i<=r1) b[d++] = a[i++];
    while(j<=r2) b[d++] = a[j++];
    for(int i = l1;i<=r2;i++) a[i] = b[i];
}
void merge_sort(int l,int r){
    if(l==r) return ;
    int mid = (l+r)/2;
    merge_sort(l,mid);
    merge_sort(mid+1,r);
    merge(l,mid,mid+1,r);
}
int main(){
    cin>>n;
    for(int i = 1;i<=n;i++) cin>>a[i];
    merge_sort(1,n);
    for(int i = 1;i<=n;i++) cout<<b[i]<<" ";
    return 0;
}