总元素个数不超过K

constexpr int K = 100000;struct L{    int v;    L *nxt, *prv;} dt[K], *mem[K]; int mc;void    init()          { for(int i = 1; i <= K; i++)   mem[i] = &dt[i];            }L       *lalloc()       { return mc < K - 1 ?               mem[--mc] : nullptr;    }void    lfree(L *&l)    { mem[mc++] = l,                    l = nullptr;            }