代码

#include <vector>
#include <algorithm>

using namespace std;

constexpr unsigned mask(unsigned x) { return x | x >> 10; }
constexpr unsigned kN = 1e7;

vector<unsigned> d[kN];
unsigned f[mask(kN) + 1];

void add(unsigned n, unsigned x) {
  for (++x; x <= n; x += x & -x) {
    ++f[mask(x)];
  }
}
unsigned sum(unsigned x) {
  unsigned s = 0;
  for (; x; x &= x - 1) {
    s += f[mask(x)];
  }
  return s;
}
void count_2d(int n, const unsigned *x, const unsigned *y, unsigned *out) {
  for (unsigned i = 0; i < n; ++i) {
    d[x[i]].push_back(i);
  }
  for (unsigned i = 0; i < n; ++i) {
    for (unsigned j : d[i]) {
      out[j] = sum(y[j]);
    }
    for (unsigned j : d[i]) {
      add(n, y[j]);
    }
  }
}

评测结果