POJ 2299 Ultra-QuickSort 逆序数 树状数组 归并排序 线段树

题目链接：http://poj.org/problem?id=2299

求逆序数的经典题，求逆序数可用树状数组，归并排序，线段树求解，本文给出树状数组，归并排序，线段树的解法。

归并排序：

#include<cstdio>
#include<iostream>
using namespace std;
#define max 500002
int arr[max],b[max];//b[]为临时序列，arr[]为待排序数列，结果在arr[]中
int tp[max];
long long cnt=0;//总逆序数
void Merge(int a[],int start,int mid,int end){
    int i =start,j=mid+1,k=start;
    while(i<=mid&&j<=end){
        if(a[i]<=a[j]){
            cnt+=j-mid-1;
            b[k++]=a[i++];
        }else{
            cnt+=j-k;
            b[k++]=a[j++];
        }
    }
    while(i<=mid){
        cnt+=end-mid;
        b[k++]=a[i++];
    }
    while(j<=end){
        b[k++]=a[j++];
    }
    for(int i=start;i<=end;i++){
        a[i]=b[i];
    }
}
void MergeSort(int a[], int start,int end){
    if(start<end){
        int mid=(start+end)/2;
        MergeSort(a,start,mid);
        MergeSort(a,mid+1,end);
        Merge(a,start,mid,end);
    }
}
int main(){
    int n;
    while(~scanf("%d",&n)&&n){
        for(int i=0;i<n;i++){
            scanf("%d",&arr[i]);
        }
        cnt=0;
        MergeSort(arr,0,n-1);
        printf("%I64d
",cnt/2);
    }
    return 0;
}

　　

树状数组：

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
#define lowbit(x) (x&(-x))
using namespace std;
const int MAX = 500005;
struct data{
    int id,val;
}num[MAX];
int n, C[MAX];
bool cmp(data a, data b){
    return a.val>b.val;
}
void add(int i){
    while(i<=n){
        C[i]+=1;
        i+=lowbit(i);
    }
}
long long sum(int i){
    long long ans = 0;
    while(i>0){
        ans+=C[i];
        i-=lowbit(i);
    }
    return ans;
}

int main(){
    while(scanf("%d",&n)&&n){
        memset(C,0,sizeof(C));
        for(int i=0;i<n;i++){
            num[i].id=i+1;
            scanf("%d",&num[i].val);
        }
        sort(num,num+n,cmp);//离散化，将数组按降序排序，再求下标的逆序数，下标的逆序数与值逆序数相等
        long long ans = 0;
        for(int i=0;i<n;i++){
            ans+=sum(num[i].id-1);//求在i前面比第i个数大的数的个数
            add(num[i].id);
        }
        printf("%I64d
", ans);
    }
    return 0;
}

 线段树( 以HDU1394 Minimum Inversion Number为例)：

#include<iostream>
#include<cstdio>
#include<cstdlib>#include<algorithm>
const int INF = 0x3F3F3F3F;
using namespace std;
#define lson l, m, rt<<1
#define rson m+1, r, rt<<1|1
#define N 5008
int sum[N<<2], a[N];
inline void pushUp(int rt){
    sum[rt] = sum[rt << 1] + sum[rt << 1 | 1];
}

int query(int a, int b, int l, int r, int rt){
    if(a <= l && b >= r){
        return sum[rt];
    }
    int m=(l + r) >> 1;
    int ret=0;
    if(a <= m){
        ret += query(a, b, lson);
    }
    if(b > m){
      ret += query(a, b, rson);
    }
    return ret;
}
void update(int x, int val, int l, int r, int rt){
    if(l == r){
        sum[rt] = val;
    }else{
    int m = (l + r)/2;
    if(x <= m){
        update(x, val, lson);
    }else{
        update(x, val, rson);
    }
    pushUp(rt);
    }
}

int main(){
    int n;
    while(~scanf("%d", &n)){
        memset(sum, 0, sizeof(sum));
        int ans = 0;
        for(int i = 0 ; i < n; i++){
            scanf("%d", &a[i]);
            a[i]++;
            ans += query(a[i] + 1, n, 1, n , 1);
            update(a[i], 1, 1, n, 1);
        }
        int tp = ans;
        for(int i = 0 ; i < n - 1; i++){
            tp += n - 2 * a[i] + 1;
            ans = min(ans , tp);
        }
        printf("%d
",ans);
    }
}