代码

// Probe: pure Fenwick in natural order (WRONG answer, just measures Fenwick time).
static unsigned tree[10000001];
void count_2d(int n, const unsigned *x, const unsigned *y, unsigned *out) {
    (void)x;
    for (int i = 0; i < n; ++i) {
        unsigned s = 0;
        for (unsigned j = y[i]; j; j &= j - 1) s += tree[j];
        out[i] = s;
        for (unsigned j = y[i] + 1; j <= (unsigned)n; j += j & -j) tree[j]++;
    }
}

评测结果