Submission #1883842

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Source Code Expand

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <vector>
using namespace std;

inline int getint()
{
	static char c;
	while ((c = getchar()) < '0' || c > '9');

	int res = c - '0';
	while ((c = getchar()) >= '0' && c <= '9')
		res = res * 10 + c - '0';
	return res;
}

const int MaxN = 200000;

int n;
int fa[MaxN + 1];

vector<int> adj[MaxN + 1];

int cir_l = 0;
int cir[MaxN + 1];

bool vis[MaxN + 1];

int f[MaxN + 1];

int li_n;
int li[MaxN + 1];

void dfs(const int &u)
{
	for (int v : adj[u])
		if (!vis[v])
			dfs(v);

	li_n = 0;
	li[li_n++] = -1;
	li[li_n++] = MaxN << 8;
	for (int v : adj[u])
		if (!vis[v])
			li[li_n++] = f[v];

	sort(li, li + li_n);
	li_n = unique(li, li + li_n) - li;
	for (int i = 0; i < li_n; ++i)
		if (li[i + 1] != li[i] + 1)
		{
			f[u] = li[i] + 1;
			break;
		}
}

inline bool check()
{
	for (int i = 1; i < cir_l; ++i)
	{
		int u = cir[i];
		int v = cir[i - 1];
		if (f[u] != f[v])
			return true;
	}

	return ~cir_l & 1;
}

int main()
{
	cin >> n;
	for (int v = 1; v <= n; ++v)
	{
		fa[v] = getint();
		adj[fa[v]].push_back(v);
	}

	int sv = 1;
	for (int i = 0; i < n; ++i)
		sv = fa[sv];

	while (!vis[sv])
	{
		cir[cir_l++] = sv;
		vis[sv] = true;
		sv = fa[sv];
	}

	for (int i = 0; i < cir_l; ++i)
		dfs(cir[i]);

	puts(check() ? "POSSIBLE" : "IMPOSSIBLE");

	return 0;
}

Submission Info

Judge Result