封锁阳光大学(Blockade Sunlight University) 题解

思路分析

按题目要求使用对应数据结构与算法模拟/递推求解，注意数据范围边界与时间复杂度。

参考代码

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

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, m;
    if (!(cin >> n >> m)) return 0;
    vector<vector<int>> adj(n + 1);
    for (int i = 0; i < m; ++i) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }
    vector<int> col(n + 1, -1), q;
    q.reserve(n);
    int ans = 0;
    for (int i = 1; i <= n; ++i) {
        if (col[i] != -1) continue;
        q.clear();
        q.push_back(i);
        col[i] = 0;
        int c0 = 0, c1 = 0;
        for (size_t head = 0; head < q.size(); ++head) {
            int u = q[head];
            if (col[u] == 0) c0++;
            else c1++;
            for (int v : adj[u]) {
                if (col[v] == -1) {
                    col[v] = col[u] ^ 1;
                    q.push_back(v);
                } else if (col[v] == col[u]) {
                    cout << "Impossible\n";
                    return 0;
                }
            }
        }
        ans += min(c0, c1);
    }
    cout << ans << "\n";
    return 0;
}