HDU 1542 —— Atlantis 【矩形面积并：扫描线】

#include <iostream>
#include <cstring>
#include <string>
#include <algorithm>
#define left rt<<1
#define right rt<<1|1
using namespace std;

const int MAXN = 100 + 5;

struct Segment {
    double l, r, h;
    int s;
    Segment() {}
    Segment(double l, double r, double h, int s):l(l),r(r),h(h),s(s) {}
    bool operator < (const Segment& a) const {
        return h < a.h;
    }
} seg[MAXN << 1];

double len[MAXN << 3];
int add[MAXN << 3];
double x[MAXN << 1];

void pushup(int rt, int l, int r)
{
    if(add[rt])    len[rt] = x[r+1] - x[l];
    else if(l == r)    len[rt] = 0;
    else    len[rt] = len[left] + len[right];
}

void update(int ql, int qr, int x, int rt, int l, int r)
{
    if(ql <= l && r <= qr) {
        add[rt] += x;
        pushup(rt, l, r);
        return;
    }
    int m = (l + r) >> 1;
    if(ql <= m)    update(ql, qr, x, left, l, m);
    if(qr > m)    update(ql, qr, x, right, m+1, r);
    pushup(rt, l, r);
}

int Bsearch(double *x, int n, double key)
{
    int l = 0, r = n - 1;
    while(l <= r) { // [l, r]
        int m = (l + r) >> 1;
        if(x[m] == key)    return m;
        else if(x[m] < key)    l = m+1;
        else    r = m-1;
    }
}

int main ()
{
    int n, segNum, xNum, kase=1;
    double x1, y1, x2, y2, ans;
    while(scanf("%d", &n) != EOF && n) {
        segNum = xNum = 0;
        for(int i=0; i<n; i++) {
            scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);
            x[xNum++] = x1;
            x[xNum++] = x2;
            seg[segNum++] = Segment(x1, x2, y1, 1);
            seg[segNum++] = Segment(x1, x2, y2, -1);
        }
        sort(x, x+xNum);
        sort(seg, seg+segNum);
        
        int cur = 1;
        for(int i=1; i<xNum; i++) {
            if(x[i] != x[i-1])    x[cur++] = x[i];
        }
        xNum = cur;
        
        // 用下面的memset替代build 
        memset(add, 0, sizeof(add));
        memset(len, 0, sizeof(len));
        
        ans = 0;
        for(int i=0; i<segNum-1; i++) {
            int l = Bsearch(x, xNum, seg[i].l);
             //这里之所以要减1，是因为每一个下标都代表的是一个区间，比如下标l其实代表的是区间[l,l+1]
            int r = Bsearch(x, xNum, seg[i].r) - 1;
            if(l <= r)    update(l, r, seg[i].s, 1, 0, xNum-1);
            ans += len[1] * (seg[i+1].h - seg[i].h);
        }
        printf("Test case #%d\nTotal explored area: %.2lf\n\n", kase++, ans);
    }
    
    return 0;
}