CS48 D BIT

统计一个点对应的和它严格右下方的点，点对数量。由于数据规模很大，不能直接上二维的前缀和，先排一维序，然后用BIT维护前缀和即可。

/** @Date    : 2017-09-14 20:17:30
  * @FileName: D.cpp
  * @Platform: Windows
  * @Author  : Lweleth (SoungEarlf@gmail.com)
  * @Link    : https://github.com/
  * @Version : $Id$
  */
#include <bits/stdc++.h>
#define LL long long
#define PII pair<int ,int>
#define MP(x, y) make_pair((x),(y))
#define fi first
#define se second
#define PB(x) push_back((x))
#define MMG(x) memset((x), -1,sizeof(x))
#define MMF(x) memset((x),0,sizeof(x))
#define MMI(x) memset((x), INF, sizeof(x))
using namespace std;

const int INF = 0x3f3f3f3f;
const int N = 1e5+20;
const double eps = 1e-8;
const LL mod = 1e9+7;
struct yuu
{
	int x, y;
};
yuu p[N];
int cmp(yuu a, yuu b)
{
	if(a.x != b.x)
	{
		return a.x < b.x;
	}
	return a.y < b.y;
}


LL t[N];

void add(int x, int v)
{
	while(x)
	{
		t[x] += v;
		t[x] %= mod;
		x -= x & (-x);
	}
}

LL sum(int x)
{
	LL res = 0;
	while(x <= N)
	{
		res += t[x];
		res %= mod;
		x += x & (-x);
	}
	return res;
}
int main()
{
	int n;
	while(cin >> n)
	{
		MMF(t);
		for(int i = 0; i < n; i++)
		{
			scanf("%d%d", &p[i].x, &p[i].y);
		}
		sort(p, p + n, cmp);
		LL ans = 0;
		for(int i = 0; i < n; i++)
		{
			int y = p[i].y;
			int x = sum(y + 1) + 1; 
			ans += x % mod;
			add(y, x);
		}
		printf("%lld
", ans % mod);
	}
    return 0;
}