你说的对，我使用基数排序，脑子都不用动一下。

详见 Luogu P4604 [WC2017] 挑战。

排序操作的时间复杂度是线性的。

#include<bits/stdc++.h>
using namespace std;
using ui=unsigned int;
istream& fin=cin;
ostream& fout=cout;
template <typename ForwardIterator,
          typename = is_unsigned<typename ForwardIterator::value_type>>
void radixSort(ForwardIterator first, ForwardIterator last) {
  constexpr ui P = 8;
  constexpr size_t T = 4;
  size_t n = distance(first, last);
  ui W = 0;
  auto *dat = new typename ForwardIterator::value_type[n];
  for (size_t i = 0; i < T; ++i, W += P) {
    array<size_t, 1u << P> cnt{};
    for_each(first, last, [&](typename ForwardIterator::reference const x) {
      ++cnt[(x >> W) & ((1u << P) - 1)];
    });
    partial_sum(cnt.begin(), cnt.end(), cnt.begin());
    rotate(cnt.begin(), prev(cnt.end()), cnt.end()), cnt[0] = 0;
    for_each(first, last, [&](typename ForwardIterator::reference const x) {
      dat[cnt[(x >> W) & ((1u << P) - 1)]++] = x;
    });
    copy(dat, dat + n, first);
  }
  delete[] dat;
}
int main(void){
    ios::sync_with_stdio(false),cin.tie(nullptr),cout.tie(nullptr);
    size_t n;fin>>n;
    vector<ui> a(n);
    for (ui& i:a) fin>>i;
    radixSort(a.begin(),a.end());
    a.erase(unique(a.begin(),a.end()),a.end());
    ui ans=0,c=1;
    for (auto it=next(a.begin());it!=a.end();++it)
        if (*it-*prev(it)==1) ++c;
        else{
            ans=max(ans,c);
            c=1;
        }
    ans=max(ans,c);
    fout<<ans;
    return 0;
}