传送阵 参考代码

#include <bits/stdc++.h>
using namespace std;

typedef long long LL;
const int maxn = 1000000 + 100;
int n, ans;
int a[maxn], fa[maxn], cnt[maxn];

void init() {
    for (int i = 1; i <= n; ++i) {
        fa[i] = i;
        cnt[i] = 1;
    }
}

int findF(int x) {
    return fa[x] == x ? x : fa[x] = findF(fa[x]);
}

void union_(int x, int y) {
    x = findF(x);
    y = findF(y);
    if (x != y) {
        fa[x] = y;
        cnt[y] += cnt[x];
    }
}

int main() {
#ifdef ExRoc
    freopen("test.txt", "r", stdin);
#endif // ExRoc
    ios::sync_with_stdio(false);

    cin >> n;
    init();
    for (int i = 1; i <= n; ++i) {
        cin >> a[i];
        union_(i, a[i]);
    }
    for (int i = 1; i < n; ++i) {
        ans = max(ans, cnt[findF(i)]);
        if (findF(i) != findF(i + 1)) {
            ans = max(ans, cnt[findF(i)] + cnt[findF(i + 1)]);
        }
    }
    cout << ans << endl;

    return 0;
}