rt,设n-m=k,我只能想到一个O(k2logkΣai+2n+nmm!Σailogn)O(k^2logk\Sigma a_i+2^n+\frac {n^m}{m!}\Sigma a_ilogn)O(k2logkΣai+2n+m!nmΣailogn)的乱搞
