50分求调

#include <vector>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#define maxn 100010
using namespace std;
//用于图操作
int n, m;
vector<int> p[maxn];
int book[maxn];

//染色
int sum[2] = {0};
int ma;
int col[maxn];

void dfs(int x, int color) {
	sum[color]++;
	col[x] = color;
	int len = p[x].size();
	for (int i = 0; i < len; i++) {
		if (book[p[x][i]]) { //访问过
			if (col[p[x][i]] == !color) { //染的色与原来相同
				continue;
			} else {
				cout << "Impossible";
				exit(0);
			}
		} else {
			book[p[x][i]] = 1;
			dfs(p[x][i], !color);
		}
	}

}

//这次不是有向图，是无向的
int main() {
	cin >> n >> m;
	int u, v;
	for (int i = 1; i <= m; i++) {
		cin >> u >> v;
		p[u].push_back(v);
		p[v].push_back(u);
	}
	for (int i = 1; i <= n; i++) {
		if (!book[i]) {
			book[i] = 1;
			dfs(i, 0);
		}
		ma = min(sum[0], sum[1]);
	}
	cout << ma;
	return 0;
}