Submission #5475398

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Source Code Expand

#include <bits/stdc++.h>

#define For(i, l, r) for (register int i = (l), i##end = int(r); i <= i##end; ++i)
#define Fordown(i, r, l) for (register int i = (r), i##end = (int)(l); i >= i##end; --i)
#define Rep(i, r) for (register int i = (0), i##end = int(r); i < i##end; ++i)
#define Set(a, v) memset(a, v, sizeof(a))
#define Cpy(a, b) memcpy(a, b, sizeof(a))
#define debug(x) cout << #x << ": " << (x) << endl

using namespace std;

template<typename T> inline bool chkmin(T &a, T b) { return b < a ? a = b, 1 : 0; }
template<typename T> inline bool chkmax(T &a, T b) { return b > a ? a = b, 1 : 0; }

inline int read() {
	int x(0), sgn(1); char ch(getchar());
	for (; !isdigit(ch); ch = getchar()) if (ch == '-') sgn = -1;
	for (; isdigit(ch); ch = getchar()) x = (x * 10) + (ch ^ 48);
	return x * sgn;
}

void File() {
#ifdef zjp_shadow
	freopen ("F.in", "r", stdin);
	freopen ("F.out", "w", stdout);
#endif
}

const int N = 2e5 + 1e3;

vector<int> G[N]; int mex[N], n, p[N];

bool vis[N], cir[N], app[N];

void Dfs(int u) {
	for (int v : G[u]) if (!cir[v]) Dfs(v);
	for (int v : G[u]) if (!cir[v]) app[mex[v]] = true;
	while (app[mex[u]]) ++ mex[u];
	for (int v : G[u]) app[mex[v]] = false;
}

int main () {

	File();

	n = read();
	For (i, 1, n)
		G[p[i] = read()].push_back(i);

	int x = 1;
	for (; !vis[x]; x = p[x]) vis[x] = true;
	for (; !cir[x]; x = p[x]) cir[x] = true;

	int minv = n + 1, maxv = 0, cnt = 0;

	For (i, 1, n) if (cir[i])
		Dfs(i), ++ cnt, chkmax(maxv, mex[i]), chkmin(minv, mex[i]);

	puts(maxv == minv && (cnt & 1) ? "IMPOSSIBLE" : "POSSIBLE");

	return 0;

}

Submission Info

Judge Result