USACO Wormholes

Code

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/*
ID: wcr19961
PROG: wormhole
LANG: C++11
*/
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <cstdlib>
#include <string>
#include <set>
using namespace std;
typedef long long LL;
const int maxn = 15;
int ans, p[maxn];
int r[maxn], st[maxn];
struct Node {
int x, y;
bool operator < (const Node& rhs) const {
return y < rhs.y || (y == rhs.y && x < rhs.x);
}
} a[maxn];
bool dfs2(int u, int fa, int n) {
if (r[p[u]] == -1) {
return false;
}
st[u] = true;
if (st[r[p[u]]] || dfs2(r[p[u]], u, n)) {
return true;
}
st[u] = false;
return false;
}
bool check(int n) {
memset(st, 0, sizeof st);
for (int i = 0; i < n; ++i) {
if (r[i] == p[i]) {
return true;
}
if (dfs2(i, -2, n)) {
return true;
}
}
return false;
}
void dfs(int d, int n) {
if (d == n) {
if (check(n)) {
++ans;
}
return;
}
if (p[d] == -1) {
for (int i = d + 1; i < n; ++i) {
if (p[i] == -1) {
p[i] = d;
p[d] = i;
dfs(d + 1, n);
p[d] = p[i] = -1;
}
}
} else {
dfs(d + 1, n);
}
}
int main() {
freopen("wormhole.in", "r", stdin);
freopen("wormhole.out", "w", stdout);
int n;
scanf("%d", &n);
for (int i = 0; i < n; ++i) {
scanf("%d%d", &a[i].x, &a[i].y);
p[i] = r[i] = -1;
}
sort(a, a + n);
for (int i = 1; i < n; ++i) {
if (a[i - 1].y == a[i].y) {
r[i - 1] = i;
}
}
dfs(0, n);
printf("%d\n", ans);
return 0;
}

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