SCP-S模拟56 C题

#include <bits/stdc++.h>

const int D = 1e9 + 7;
int n, c[2510], pos[2510][2510];
bool used[2510];

struct Bit {
  int t[2510];
  void add(int x, int d) {
    for (; x <= 2510; x += x & -x) t[x] += d;
  }
  int ask(int x) {
    int ret = 0;
    for (; x; x -= x & -x) ret += t[x];
    return ret;
  }
  void clear() {
    for (int i = 1; i <= 2510; ++i) t[i] = 0;
  }
} siz, len;

inline int read() {
  int a = 0, b = 0;
  char c = getchar();
  while (!isdigit(c)) b = c == ‘-‘ ? 1 : 0, c = getchar();
  while (isdigit(c)) a = a * 10 + c - ‘0‘, c = getchar();
  return b ? -a : a;
}

void modify(int x) {
  if (!used[x]) {
    used[x] = true;
    siz.add(x, 1), len.add(x, x);
  }
}

signed main() {
  n = read();
  for (int i = 1, x, y; i <= n; ++i) x = read(), y = read(), pos[x][++c[x]] = y;
  for (int i = 1; i <= 2500; ++i)
    std::sort(pos[i] + 1, pos[i] + 1 + c[i]), pos[i][c[i] + 1] = 2501;
  long long ans = 0;
  for (int i = 1; i <= 2500; ++i) {
    if (!c[i]) continue;
    siz.clear(), len.clear();
    for (int j = 1; j    2500; ++j) used[j] = false;
    for (int j = 1; j <= c[i]; ++j) modify(pos[i][j]);
    for (int j = i - 1; j; --j) {
      if (!c[j]) continue;
      int p1 = 1, p2 = 1, cur = std::max(pos[i][1], pos[j][1]);
      for (int k = 1; k <= c[j]; ++k) modify(pos[j][k]);
      while (pos[i][p1 + 1] <= cur) ++p1;
      while (pos[j][p2 + 1] <= cur) ++p2;
      while (p1 <= c[i] && p2 <= c[j]) {
        int mn = std::min(pos[i][p1], pos[j][p2]);
        int trend = std::min(pos[i][p1 + 1], pos[j][p2 + 1]);
        (ans += (long long) (i - j) *
                ((long long) (len.ask(trend - 1) - len.ask(cur - 1)) * siz.ask(mn) -
                 (long long) (siz.ask(trend - 1) - siz.ask(cur - 1)) * len.ask(mn))) %= D;
        cur = trend;
        if (pos[i][p1 + 1] <= cur) ++p1;
        if (pos[j][p2 + 1] <= cur) ++p2;
      }
    }
  }
  return !printf("%lld\n", ans);
}