poj2299

树状数组

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
using namespace std;

#define maxn 500005

struct Elem
{
    int p, v;
}s[maxn];

int f[maxn];
int pos[maxn];
int ar[maxn];

bool operator < (const Elem &a, const Elem &b)
{
    return a.v < b.v;
}

int lowb(int t)
{
    return t & (-t);
}

void add(int i, int v)
{
    for (; i < maxn; ar[i] +=v, i += lowb(i));
}

int sum(int i)
{
    int s = 0;
    for (; i > 0; s += ar[i], i -= lowb(i));
    return s;
}

int main()
{
    //freopen("t.txt", "r", stdin);
    int n;
    memset(ar, 0, sizeof(ar));
    while (scanf("%d", &n), n)
    {
        memset(ar, 0, sizeof(ar));
        for (int i = 0; i < n; i++)
        {
            scanf("%d", &f[i]);
            s[i].v = f[i];
            s[i].p = i;
        }
        sort(s, s + n);
        for (int i = 0; i < n; i++)
            pos[s[i].p] = i + 1;
        long long ans = 0;
        for (int i = n - 1; i >= 0; i--)
        {
            ans += sum(pos[i] - 1);
            add(pos[i], 1);
        }
        printf("%lld\n", ans);
    }
    return 0;
}