SGU 180. Inversions 離散 + 樹狀數組

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int maxn = 70000;
int n, aa[maxn], c[maxn];
struct node
{
    int num;
    int ix;
}a[maxn];

bool cmp(node a, node b)
{
    return a.num < b.num;
}

int lowbit(int x)
{
    return x&-x;
}

void add(int x, int d)
{
    while(x <= n)
    {
        c[x] += d;
        x += lowbit(x);
    }
}

int sum(int x)
{
    int ret = 0;
    while(x > 0)
    {
        ret += c[x];
        x -= lowbit(x);
    }
    return ret;
}

int main()
{
    while(scanf("%d", &n) != EOF)
    {
        for(int i = 1; i <= n; i++)
        {
            scanf("%d", &a[i].num);
            a[i].ix = i;
        }
        sort(a+1, a+1+n, cmp);
        aa[a[1].ix] = 1;
        for(int i = 2; i <= n; i++)
        {
            if(a[i].num != a[i-1].num) aa[a[i].ix] = i;
            else aa[a[i].ix] = aa[a[i-1].ix];
        }

        memset(c, 0, sizeof(c));
        long long ans = 0;
        for(int i = 1; i <= n; i++)
        {
            add(aa[i], 1);
            ans += sum(n) - sum(aa[i]);
        }
        printf("%lld\n", ans);
    }
}