NYOJ117 求逆序数

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http://acm.nyist.net/JudgeOnline/problem.php?pid=117

用树状数组做，一定要注意sort 的特性，当元素的值很大的时候sort 是不稳定的，所以我们要把sort 变成稳定的排序

#include<stdio.h>#include<iostream>#include<cmath>#include<cstring>#include<algorithm>using namespace std;int c[1000010],b[1000010],n;struct A{    int v,index;    A(){}    A(int v,int index):v(v),index(index){}    bool operator < (const A& rhs) const//排序的时候先按元素排，然后按照下表排，把sort 变成稳定的排序    {        if(v != rhs.v)        return v < rhs.v;        else return index < rhs.index;    }}a[1000010];int lowbit(int x){    return x & (-x);}void add(int x,int y){    while(x <= n)    {        c[x] += y;        x += lowbit(x);    }}int query(int x){    int ans = 0;    while(x > 0)    {        ans += c[x];        x -= lowbit(x);    }    return ans;}int main(){    int m,T;    cin>>T;    while(T--)    {        cin>>n;        memset(c,0,sizeof(c));        //memset(b,0,sizeof(b));        int i;        long long res = 0;        for(i = 1; i <= n; i++)        {            cin>> a[i].v;            a[i].index = i;        }        sort(a+1,a+n+1);        for(i = 1; i <= n; i++)        b[a[i].index] = i;        for(i = 1; i <= n; i++)        //cout<<b[i]<<"***"<<endl;        for(i = 1; i <= n; i++)        {                        res += i -1 - query(b[i]-1);            add(b[i],1);        }        cout<<res<<endl;    }    return 0;}

归并排序：

#include<stdio.h>int a[1000005],temp[1000005];long long sum;void merge(int l ,int mid,int r){    int i = l, j = mid+1,k = l;    while(i <= mid && j <= r)    {        if(a[i] <= a[j])        {            temp[k++] = a[i++];        }        else        {            temp[k++] = a[j++];            sum += mid - i+1;        }    }    while(i <= mid) temp[k++] = a[i++];    while(j <= r) temp[k++] = a[j++];    for(i = l; i <= r ;i++)    a[i] = temp[i];}void msort(int l, int r){    if(l == r) return ;    int mid = (l + r)/2;    msort(l,mid);    msort(mid+1,r);    merge(l,mid,r);}int main(){    int i,n,T;    scanf("%d",&T);    while(T--)    {        scanf("%d",&n);        for(i = 0;i < n; i ++)            scanf("%d",&a[i]);        sum = 0;        msort(0,n - 1);        printf("%lld\n",sum);    }    return 0;}

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