Luogu P1330 封锁阳光大学 (黑白染色)

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<string>
#include<stack>
#include<queue>
#include<deque>
#include<set>
#include<vector>
#include<map>
#include<functional>
    
#define fst first
#define sc second
#define pb push_back
#define mem(a,b) memset(a,b,sizeof(a))
#define lson l,mid,root<<1
#define rson mid+1,r,root<<1|1
#define lc root<<1
#define rc root<<1|1
#define lowbit(x) ((x)&(-x)) 

using namespace std;

typedef double db;
typedef long double ldb;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> PI;    
typedef pair<ll,ll> PLL;

const db eps = 1e-6;
const int mod = 1e9+7;
const int maxn = 2e6+100;
const int maxm = 2e6+100;
const int inf = 0x3f3f3f3f;
const db pi = acos(-1.0);
vector<int>v[maxn];
int vis[maxn];
int c[maxn];
int sum[2];
bool dfs(int i, int cl){
    if(vis[i]==1){
        if(cl==c[i])return true;
        else return false;
    }
    vis[i] = 1;
    c[i] = cl;
    sum[cl]++;
    int sz = v[i].size();
    bool ans = true;
    for(int j = 0; j < sz; j++){
        ans = ans && dfs(v[i][j], cl^1);
    }
    return ans;
}
int main() {
    int n, m;
    scanf("%d %d", &n, &m);
    for(int i = 0; i < m; i++){
        int x, y;
        scanf("%d %d", &x, &y);
        v[x].pb(y);
        v[y].pb(x);
    }
    mem(vis, 0);
    int ans = 0;
    for(int i = 0; i < n; i++){
        if(vis[i]==1)continue;
        sum[0] = sum[1] = 0;
        if(!dfs(i, 0)){
            printf("Impossible");
            return 0;
        }
        ans += min(sum[0], sum[1]);
    }
    printf("%d", ans);
    return 0;
}
/*
4
2.
2 4
0 0 1 1
2 4
0 1 0 2
2 3
0 1 2
2 4
0 0 0 1
*/