题解（不建议食用）

前面忘了，后面忘了，反正过了

#include <bits/stdc++.h>
#define ll long long
#define endl '\n'
#define pii pair<int, int>
using namespace std;
const int maxn = 2e5 + 5;
int n;
int a[maxn];
int cnt[10005];
int main() {
    cin >> n;
    int mx1 = 0, mx2 = 0;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        cnt[a[i]]++;
        if (a[i] > mx1) {
            mx2 = mx1;
            mx1 = a[i];
        }
        else if (a[i] > mx2) {
            mx2 = a[i];
        }
    }
    int ans = 0;
    for (int i = 2; i <= mx1 + mx2; i++) {
        int now = 0;
        for (int j = 1; j <= i / 2; j++) {
            int k = i - j;
            if (k < 1 || k > 5000) continue;
            if (j * 2 != i)
                now += min(cnt[j], cnt[k]);
            else now += cnt[j] / 2;
        }
        ans = max(now, ans);
    }
    cout << ans * 2 << endl;
}