【LG2154】[SDOI2009]虔诚的墓主人
题面
题解
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可以发现，对于某一个点，它的答案就是上下左右几个组合数乘起来。
这样直接做复杂度显然爆炸，考虑怎么优化这个东西。
我们可以固定左右，在某两个左右之间维护上下有多少个，这样子的话左右的贡献就是不变的，而且你最多只会变化\(O(n)\)次左右边界，复杂度有保证。
这样的话，每次查询一个左右边界内上下的贡献，用线段树维护即可。(描述可能有些模糊，具体详见代码)
代码
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <vector>
using namespace std;
inline int gi() {
register int data = 0, w = 1;
register char ch = 0;
while (!isdigit(ch) && ch != '-') ch = getchar();
if (ch == '-') w = -1, ch = getchar();
while (isdigit(ch)) data = 10 * data + ch - '0', ch = getchar();
return w * data;
}
const int MAX_N = 1e5 + 5;
#define lson (o << 1)
#define rson (o << 1 | 1)
int sum[MAX_N << 2];
void modify(int o, int l, int r, int pos, int v) {
if (l == r) return (void)(sum[o] = v);
int mid = (l + r) >> 1;
if (pos <= mid) modify(lson, l, mid, pos, v);
else modify(rson, mid + 1, r, pos, v);
sum[o] = sum[lson] + sum[rson];
}
int query(int o, int l, int r, int ql, int qr) {
if (ql <= l && r <= qr) return sum[o];
int mid = (l + r) >> 1, res = 0;
if (ql <= mid) res += query(lson, l, mid, ql, qr);
if (qr > mid) res += query(rson, mid + 1, r, ql, qr);
return res;
}
int W, H, N, K;
int x[MAX_N], y[MAX_N], ox[MAX_N], oy[MAX_N], tx, ty;
vector<int> vec[MAX_N];
int C[MAX_N][15], c1[MAX_N], c2[MAX_N];
bool cmp(const int &l, const int &r) { return y[l] < y[r]; }
int main () {
#ifndef ONLINE_JUDGE
freopen("cpp.in", "r", stdin);
#endif
W = gi(), H = gi(), N = gi();
for (int i = 1; i <= N; i++) x[i] = ox[i] = gi(), y[i] = oy[i] = gi();
K = gi();
sort(&ox[1], &ox[N + 1]), sort(&oy[1], &oy[N + 1]);
tx = unique(&ox[1], &ox[N + 1]) - ox - 1;
ty = unique(&oy[1], &oy[N + 1]) - oy - 1;
for (int i = 1; i <= N; i++) {
x[i] = lower_bound(&ox[1], &ox[tx + 1], x[i]) - ox;
y[i] = lower_bound(&oy[1], &oy[ty + 1], y[i]) - oy;
}
for (int i = 1; i <= N; i++) vec[x[i]].push_back(i), c1[y[i]]++;
for (int i = 1; i <= tx; i++) vec[i].push_back(N + 1);
y[N + 1] = ty + 1;
for (int i = 1; i <= tx; i++) sort(vec[i].begin(), vec[i].end(), cmp);
for (int i = 0; i <= N; i++) C[i][0] = 1;
for (int i = 1; i <= N; i++)
for (int j = 1; j <= min(i, K); j++)
C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
int ans = 0;
for (int i = 1; i <= tx; i++) {
int res = 0, sz = vec[i].size();
for (int j = 0; j < sz; j++) {
int l = j ? y[vec[i][j - 1]] : 0, r = y[vec[i][j]];
if (l + 1 <= r - 1) ans += res * query(1, 1, ty, l + 1, r - 1);
if (j == sz - 1) break;
res = C[j + 1][K] * C[sz - j - 2][K];
++c2[r];
modify(1, 1, ty, r, C[c2[r]][K] * C[c1[r] - c2[r]][K]);
}
}
if (ans < 0) ans += 2147483648ll;
printf("%d\n", ans);
return 0;
}