题目来源：HDU 1542 Atlantis

题意：给你一些矩形（左下角和右上角）求面积

思路：参考here这个超赞的 一看就懂了

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cctype>
using namespace std;
const int maxn = 210;
struct node
{
	double l, r, h;
	int s, val;
	node(){}
	node(double a, double b, double c, int d) : l(a), r(b), h(c), s(d) {} 
}seg[2*maxn];
double sum[maxn<<2];
double X[2*maxn];
int len[maxn<<2];
struct Point
{
	double x;
	int id, v;
}b[2*maxn];
bool cmp(node a, node b)
{
	return a.h < b.h;
}
bool cmp1(Point a, Point b)
{
	return a.x < b.x;
}

bool cmp2(Point a, Point b)
{
	return a.id < b.id;
}
void pushup(int rt, int l, int r)
{
	if(len[rt])
		sum[rt] = X[r+1] - X[l];
	else if(l == r)
		sum[rt] = 0;
	else
		sum[rt] = sum[rt<<1] + sum[rt<<1|1];
}
void update(int x, int y, int l, int r, int rt, int c)
{
	
	if(x == l && y == r)
	{
		len[rt] += c;
		pushup(rt, l, r);
		return;	
	}
	int m = (l + r) >> 1;
	if(y <= m)
		update(x, y, l, m, rt<<1, c);
	else if(x > m)
		update(x, y, m+1, r, rt<<1|1, c);
	else
	{
		update(x, m, l, m, rt<<1, c);
		update(m+1, y, m+1, r, rt<<1|1, c);
	}
	pushup(rt, l, r); 
}
int main()
{
	int cas = 1;
	int n;
	while(scanf("%d", &n) && n)
	{
		int m = 0, k = 1;
		for(int i = 0; i < n; i++)
		{
			double a, b, c, d;
			scanf("%lf %lf %lf %lf", &a, &b, &c, &d);
			seg[m++] = node(a, c, b, 1);
			seg[m++] = node(a, c, d, -1);
			X[k++] = a;
			X[k++] = c;
		}
		sort(seg, seg+m, cmp);
		for(int i = 0; i < m; i++)
		{
			b[i<<1].x = seg[i].l;
			b[i<<1].id = i<<1;
			b[i<<1|1].x = seg[i].r;
			b[i<<1|1].id = i<<1|1;
		}
		sort(b, b+2*m, cmp1);
		int num = 1;
		double now = b[0].x;
		for(int i = 0; i < 2*m; i++)
		{
			if(b[i].x != now)
			{
				num++;
				now = b[i].x;
			}
			b[i].v = num;
		}
		sort(b, b+2*m, cmp2);
		double s = 0;
		memset(sum, 0, sizeof(sum));
		memset(len, 0, sizeof(len));
		sort(X+1, X+k);
		k = 1;
		for(int i = 2; i <= m; i++)
		{
			if(X[i] != X[i-1])
				X[++k] = X[i];
		}
		for(int i = 0; i < m-1; i++)
		{
			int l = b[i<<1].v;
			int r = b[i<<1|1].v-1;
			if(l <= r)
				update(l, r, 1, num, 1, seg[i].s);
			s += sum[1] * (seg[i+1].h - seg[i].h);
		}
		printf("Test case #%d\n", cas++);
		printf("Total explored area: %.2f\n\n", s);
	}
	return 0;
}