传送阵（用STL库极限压缩时间）

解题思路:

注意事项:

参考代码:

#include<bits/stdc++.h>

using namespace std;

const int N = 1e6 + 5;

int mp[N];

int n, ans,k;

bool vis[N];

struct

{

int val, flag;

}v[N];

map<int, int> f;

typedef pair<int, int> pll;

map<pll, int> fff;

pll ffff;

void dfs(int x, int num, int ff)

{

if (vis[x])

{

f[ff] = num - 1;

return;

}

vis[x] = true;

v[x].flag = ff;

dfs(mp[x], num + 1, ff);

}

int main()

{

cin >> n;

for (int i = 1; i <= n; i++)

{

cin >> mp[i];

}

for (int i = 1; i <= n; i++)

{

dfs(i, 1, i);

}

for (int i = 2; i <= n-1; i++)

{

pll f1, f2;

f1.first = v[i - 1].flag; f1.second = v[i].flag;

f2.first = v[i + 1].flag; f2.second = v[i].flag;

if (v[i - 1].flag != v[i].flag&&!fff[f1])

{

map<int, int>::iterator it1;

it1 = f.find(v[i - 1].flag);

map<int, int>::iterator it2;

it2 = f.find(v[i].flag);

ans = max(it1->second + it2->second, ans);

fff[f1]++;

int x = f1.second;

f1.first = f1.second;

f1.second = x;

fff[f1]++;

}

if (v[i + 1].flag != v[i].flag&&!fff[f2])

{

map<int, int>::iterator it1;

it1 = f.find(v[i + 1].flag);

map<int, int>::iterator it2;

it2 = f.find(v[i].flag);

ans = max(it1->second + it2->second, ans); fff[f1]++;

int x = f2.second;

f2.first = f2.second;

f2.second = x;

fff[f2]++;

}

}

cout << ans;

}