快速排序 & 归并排序模板

快速排序+随机化：

#include<bits/stdc++.h>
#define Rand(a,b) rand()%(b-a)+a
using namespace std;

const int maxn=200010;
int a[maxn];

int read(){
	int Value=0,Base=1;char Ch=getchar();
	for(;!isdigit(Ch);Ch=getchar())if(Ch=='-')Base=-1;
	for(;isdigit(Ch);Ch=getchar())Value=Value*10+(Ch^'0');
	return Value*Base;
}

void Quick_sort(int l,int r){
	if(l>=r)return ;
	int key=a[Rand(l,r)],head=l,tail=r;
	while(head<=tail){
		while(a[head]<key)head++;
		while(a[tail]>key)tail--;
		if(head<=tail)
			swap(a[head++],a[tail--]);
	}
	Quick_sort(l,tail);Quick_sort(head,r);
}

int main( ){
	int m,n,j,k,i;
	srand((int)time(NULL));
	n=read();
	for(i=1;i<=n;i++)
		a[i]=read();
	Quick_sort(1,n);
	for(i=1;i<=n;i++)
		printf("%d ",a[i]);
	puts("");
	return 0;
}

归并排序+逆序对：

#include<bits/stdc++.h>
using namespace std;

const int maxn=200010;
int a[maxn],ans,t[maxn];

int read(){
	int Value=0,Base=1;char Ch=getchar();
	for(;!isdigit(Ch);Ch=getchar())if(Ch=='-')Base=-1;
	for(;isdigit(Ch);Ch=getchar())Value=Value*10+(Ch^'0');
	return Value*Base;
}

void Merge_sort(int l,int r){
	if(l>=r)return ;
	int mid=(l+r)>>1,i=l,j=mid+1,k=l;
	Merge_sort(l,mid);Merge_sort(mid+1,r);
	while(i<=mid && j<=r){
		if(a[i]<=a[j])t[k++]=a[i++];
		else t[k++]=a[j++],ans+=mid-i+1;
	}
	while(i<=mid)t[k++]=a[i++];
	while(j<=r)t[k++]=a[j++];
	for(i=l;i<=r;i++)a[i]=t[i];
}

int main( ){
	int m,n,j,k,i;
	n=read();
	for(i=1;i<=n;i++)a[i]=read();
	Merge_sort(1,n);
	printf("%d\n",ans);
	/*
	for(i=1;i<=n;i++)
		printf("%d ",a[i]);
	puts("");
	*/
	return 0;
}

思路比较简单，都是分治法的体现，可进行比较