題目鏈接:http://poj.org/problem?id=2482
代碼風格:http://www.notonlysuccess.com/index.php/segment-tree-complete/
題目大意:一個平面有n個星星,每個星星有都有不同的亮度,求用一個矩形的框框框星星,求框住的星星亮度總和最大是多少(右邊 和上面邊界的星星不計算在內)。
用到算法:掃描線, 離散化
題目思路:從每個點往右上方延伸得到一個h * w的矩形,自己模擬一下,應該可以得到~~這是一個很明顯的,具有掃描線特性的圖案,把每個矩形的下邊界加入線段樹,並且,該線段賦值爲v, 上邊界加入線段樹,並且,該線段賦值爲-v,線段樹維護總的區間內最大的覆蓋次數,把每次的msum[1]與maxn作比較,能更新就更新。
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
#define lson l, m, rt << 1
#define rson m+1, r, rt << 1 | 1
#define mid int m = (l+r) >> 1
#define LL __int64
struct Seg
{
int s;
unsigned l, r, h;
Seg(){}
Seg(unsigned a, unsigned b, unsigned c, int d) : l(a), r(b), h(c), s(d){}
bool operator < (const Seg &cmp) const
{
if(h == cmp.h) return s < cmp.s;
return h < cmp.h ;
}
}p[898989];
unsigned x[456789];
int msum[456456];
int add[456456];
int maxz(int a, int b)
{
return a < b ? b : a;
}
void PushUp(int rt)
{
msum[rt] = maxz(msum[rt << 1 ] , msum[rt << 1 | 1]);
}
void PushDown(int rt)
{
if(add[rt])
{
add[rt << 1] += add[rt];
add[rt << 1 | 1] += add[rt];
msum[rt << 1 ] += add[rt];
msum[rt << 1 | 1] += add[rt];
add[rt] = 0;
}
}
void update(int L, int R, int k, int l, int r, int rt)
{
if(L <= l && r <= R)
{
add[rt] += k;
msum[rt] += k;
return ;
}
mid ;
PushDown(rt);
if(L <= m)
update(L, R, k, lson);
if(m < R)
update(L, R, k, rson);
PushUp(rt);
}
int bin(unsigned key, int r)
{
int l = 0;
while(l <= r)
{
mid ;
if(key == x[m])
return m;
if(x[m] < key)
l = m+1;
else
r = m-1;
}
return -1;
}
int main()
{
int n;
unsigned a, b, w, h;
int c, d;
while(scanf("%d%d%d", &n, &w, &h) != EOF)
{
int i, k = 0;
for(i = 1; i <= n; i ++)
{
scanf("%d%d%d", &a, &b, &c);
x[k] = a;
p[k ++] = Seg(a, a+w, b, c);
x[k] = a+w;
p[k ++] = Seg(a, a+w, b+h, -c);
}
sort(x, x+k);
sort(p, p+k);
int m = 1;
for(i = 1; i < k; i ++ )
{
if(x[i] != x[i-1])
x[m ++] = x[i];
}
int maxn = 0;
memset(add, 0, sizeof(add));
memset(msum, 0, sizeof(msum));
for(i = 0; i < k-1; i ++)
{
int ll = bin(p[i].l, m-1);
int rr = bin(p[i].r, m-1)-1;
if(ll <= rr)
update(ll, rr, p[i].s, 0, m-1, 1);
if(maxn < msum[1])
maxn = msum[1];
}
printf("%d\n", maxn);
}
return 0;
}