A14 归并排序 逆序对

视频链接：56 归并排序 逆序对_哔哩哔哩_bilibili

 Luogu P1177 【模板】排序

 

#include <iostream>
using namespace std;

const int N = 100010;
int n,a[N],b[N]; //b为辅助数组

void msort(int l, int r){
  if(l==r) return;
  int mid=l+r>>1;
  msort(l,mid);
  msort(mid+1,r); //拆分

  int i=l,j=mid+1,k=l; //合并
  while(i<=mid && j<=r){
    if(a[i]<=a[j]) b[k++]=a[i++];
    else b[k++]=a[j++];
  }
  while(i<=mid) b[k++]=a[i++];
  while(j<=r) b[k++]=a[j++];
  for(i=l; i<=r; i++) a[i]=b[i];
}

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

 

Luogu P1908 逆序对

 

#include <iostream>
using namespace std;

int n,a[500010],b[500010];
long long res;

void merge(int l,int r){
  if(l>=r) return;
  int mid=l+r>>1;
  merge(l,mid); 
  merge(mid+1,r); //拆分
  
  int i=l,j=mid+1,k=l; //合并
  while(i<=mid && j<=r){
    if(a[i]<=a[j]) b[k++]=a[i++];
    else b[k++]=a[j++], res+=mid-i+1;
  }
  while(i<=mid) b[k++]=a[i++];
  while(j<=r) b[k++]=a[j++];
  for(i=l; i<=r; i++) a[i]=b[i];
}

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

 

Luogu P1966 [NOIP2013 提高组] 火柴排队

#include<cstdio>
#include<algorithm>
using namespace std;

struct node{
  int num,ord;
  bool operator<(node b){
    return num<b.num;
  }
}f[100010],s[100010];
int a[100010],b[100010],ans;

void merge(int l,int r){
  if(l>=r) return;
  int mid=(l+r)/2;
  merge(l,mid);
  merge(mid+1,r);
  int i=l,j=mid+1,k=l;
  while(i<=mid&&j<=r){
    if(a[i]>a[j]){
      b[k++]=a[j++];
      ans+=mid-i+1;
      ans%=99999997;
    }
    else b[k++]=a[i++];
  }
  while(i<=mid) b[k++]=a[i++];
  while(j<=r) b[k++]=a[j++];
  for(int i=l;i<=r;i++) a[i]=b[i];
}
int main(){
  int n; scanf("%d",&n);
  for(int i=1;i<=n;i++){
    scanf("%d",&f[i].num);
    f[i].ord=i;
  }
  for(int i=1;i<=n;i++){
    scanf("%d",&s[i].num);
    s[i].ord=i;
  }
  sort(f+1,f+n+1);
  sort(s+1,s+n+1);
  for(int i=1;i<=n;i++)
    a[f[i].ord]=s[i].ord;
  merge(1,n);
  printf("%d",ans);
  return 0;
}