题意：
判断给出的线段是否组成了多边形。
Solution：
简单题，并查集+hash
这里用map实现
code
#include <iostream>
#include <cstdio>
#include <utility>
#include <map>
using namespace std;
typedef pair<int, int> ii;
map<ii, int> f;
int fa[400009];
int n, xa, ya, xb, yb, cnt;
int Find (int x) {
if (fa[x] != x) return fa[x] = Find (fa[x]);
return x;
}
int main() {
scanf ("%d", &n);
for (int i = 1; i <= 2 * n; i++) fa[i] = i;
for (int i = 1; i <= n; i++) {
scanf ("%d %d %d %d", &xa, &ya, &xb, &yb);
ii tem1 = make_pair (xa, ya), tem2 = make_pair (xb, yb);
if (f.count (tem1) == 0 ) f[tem1] = ++cnt;
if (f.count (tem2) == 0 ) f[tem2] = ++cnt;
int x = Find (f[tem1]), y = Find (f[tem2]);
if (x != y)
fa[x] = y;
else {
cout << i << endl;
return 0;
}
}
cout << 0 << endl;
return 0;
}