归并排序主要的思想是分治和合并，合并我觉得挺好理解的，分治是用递归实现的感觉不太好理解，我就贴一个模板，拿着就能用了。要是像仔细学习了解归并排序的话可以看下这篇文章传送门，感觉讲的不能再详细了。。。


归并排序模板：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <algorithm>
#define ll long long
using namespace std;
ll t[1000005];
ll a[1000005];
int n;
ll ans;

void merge(int left,int mid,int right){
  int i = left, j = mid + 1, k = left;
  while(i <= mid && j <= right){
    if(a[i] <= a[j]){
      t[k++] = a[i++];
    }
    else{
      t[k++] = a[j++];
    }
  }
  while(i <= mid){
    t[k++] = a[i++];
  }
  while(j <= right){
    t[k++] = a[j++];
  }
  for(int i=left;i<=right;i++){
    a[i] = t[i];
  }
}

void mergesort(int left,int right){
  if(left < right){
    int mid = (left + right) / 2;
    mergesort(left,mid);
    mergesort(mid+1,right);
    merge(left,mid,right);
  }
}

int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
      scanf("%d",&a[i]);
    }
    ans = 0;
    mergesort(1,n);
    printf("%lld\n",ans);
  return 0;
}