POJ - 2299 Ultra-QuickSort（归并排序）

https://vjudge.net/problem/POJ-2299

题意

求对于给定的无序数组，求出经过最少多少次相邻元素的交换之后，可以使数组从小到大有序。

分析

很明显是求逆序对的数目，那就要想到归并排序了。在归并过程中计算逆序对。

#include<iostream>
#include<cmath>
#include<cstring>
#include<queue>
#include<vector>
#include<cstdio>
#include<algorithm>
#include<map>
#include<set>
#define rep(i,e) for(int i=0;i<(e);i++)
#define rep1(i,e) for(int i=1;i<=(e);i++)
#define repx(i,x,e) for(int i=(x);i<=(e);i++)
#define X first
#define Y second
#define PB push_back
#define MP make_pair
#define mset(var,val) memset(var,val,sizeof(var))
#define scd(a) scanf("%d",&a)
#define scdd(a,b) scanf("%d%d",&a,&b)
#define scddd(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define pd(a) printf("%d\n",a)
#define scl(a) scanf("%lld",&a)
#define scll(a,b) scanf("%lld%lld",&a,&b)
#define sclll(a,b,c) scanf("%lld%lld%lld",&a,&b,&c)
#define IOS ios::sync_with_stdio(false);cin.tie(0)

using namespace std;
typedef long long ll;
template <class T>
void test(T a){cout<<a<<endl;}
template <class T,class T2>
void test(T a,T2 b){cout<<a<<" "<<b<<endl;}
template <class T,class T2,class T3>
void test(T a,T2 b,T3 c){cout<<a<<" "<<b<<" "<<c<<endl;}
template <class T>
inline bool scan_d(T &ret){
    char c;int sgn;
    if(c=getchar(),c==EOF) return 0;
    while(c!='-'&&(c<'0'||c>'9')) c=getchar();
    sgn=(c=='-')?-1:1;
    ret=(c=='-')?0:(c-'0');
    while(c=getchar(),c>='0'&&c<='9') ret = ret*10+(c-'0');
    ret*=sgn;
    return 1;
}
//const int N = 1e6+10;
const int inf = 0x3f3f3f3f;
const ll INF = 0x3f3f3f3f3f3f3f3fll;
const ll mod = 1000000000;
int T;

void testcase(){
    printf("Case %d:",++T);
}

const int MAXN = 5e5+5 ;
const int MAXM = 550;
const double eps = 1e-8;
const double PI = acos(-1.0);
ll a[MAXN],tmp[MAXN];
ll ans;
int n;

void Merge(int low,int mid,int high){
    int i=low,j=mid+1,k=low;
    while(i<=mid&&j<=high){
        if(a[i]<=a[j]){
            tmp[k++]=a[i++];
        }else{
            ans += j-k;
            tmp[k++] = a[j++];
        }
    }
    while(i<=mid) tmp[k++] = a[i++];
    while(j<=high) tmp[k++] = a[j++];
    for(i=low;i<=high;++i){
        a[i]=tmp[i];
    }
}
void mergeSort(int a,int b){
    if(a<b){
        int mid =(a+b)>>1;
        mergeSort(a,mid);
        mergeSort(mid+1,b);
        Merge(a,mid,b);
    }
}

int main() {
#ifdef LOCAL
    freopen("in.txt","r",stdin);
#endif // LOCAL
    while(~scanf("%d",&n)&&n){
        ans=0;
        for(int i=0;i<n;i++) scanf("%lld",&a[i]);
        mergeSort(0,n-1);
        printf("%lld\n",ans);
    }
    return 0;
}