hdu 1166 敌兵布阵 树状数组

题目链接：http://acm.hdu.edu.cn/showproblem.php?pid=1166

裸的树状数组

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
#include <cmath>
#include <vector>
#include <stack>
#include <set>
#include <map>
#include <algorithm>
#include <sstream>

using namespace std;

typedef long long ll;

const int maxn = 500010;

struct pre
{
    int num, id;
    bool operator < (const pre &b) const
    {
        return num < b.num;
    }
}p[maxn];

int a[maxn], bit[maxn], n;

int sum(int i)
{
    int s = 0;
    while(i > 0)
    {
        s += bit[i];
        i -= i & -i;
    }
    return s;
}

void add(int i, int x)
{
    while(i <= n)
    {
        bit[i] += x;
        i += i & -i;
    }
}

int main()
{
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
#endif

    while(scanf("%d", &n) == 1 && n)
    {
        for(int i = 0; i < n; i++)
        {
            scanf("%d", &p[i].num);
            p[i].id = i;
        }

        sort(p, p + n);

        for(int i = 1; i <= n; i++)
        {
            a[p[i-1].id] = i;
        }

        ll ans = 0;

        for(int j = 0; j < n; j++)
        {
            ans += j - sum(a[j]);
            add(a[j], 1);
        }

        printf("%I64d
", ans);

        for(int i = 0; i <= n; i++)
        {
            bit[i] = 0;
        }
    }

    return 0;
}