https://codeforc.es/contest/1176/problem/E

久了不写bfs了。一开始用dfs写，的确用dfs是很有问题的，一些奇怪的情况就会导致多染一些色。

注意无向图的边要开双倍。

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;int n, m;struct Edge {int u, v;int next;Edge() {}Edge(int u, int v, int next): u(u), v(v), next(next) {}
};struct Graph {Edge edge[400005];int head[200005], etop;void init(int n) {etop = 0;memset(head + 1, -1, sizeof(head[1])*n);}void add_edge(int u, int v) {edge[++etop] = Edge(u, v, head[u]);head[u] = etop;edge[++etop] = Edge(v, u, head[v]);head[v] = etop;}
} g;bool visited[200005];
bool color[200005];
int dis[200005];int que[200005];
int qhead, qtail;int cntodd, cnteven;void enqueue(int id, int d) {if(visited[id])return;visited[id] = true;dis[id] = d;if(d & 1)cntodd++;elsecnteven++;que[++qtail] = id;
}void bfs() {memset(visited + 1, false, sizeof(visited[1])*n);qhead = 1, qtail = 0;cntodd = 0, cnteven = 0;enqueue(1, 0);while(qhead <= qtail) {int u = que[qhead++];for(int i = g.head[u]; i != -1; i = g.edge[i].next) {enqueue(g.edge[i].v,dis[u]+1);}}
}int main() {
#ifdef Yinkufreopen("Yinku.in", "r", stdin);//freopen("Yinku.out", "w", stdout);
#endif // Yinkuint t;while(~scanf("%d", &t)) {while(t--) {scanf("%d%d", &n, &m);g.init(n);for(int i = 1; i <= m; i++) {int u, v;scanf("%d%d", &u, &v);g.add_edge(u, v);}bfs();int ch = 0;if(cntodd <= cnteven)ch = 1;int cnt = 0, last = -1;for(int i = 1; i <= n; i++) {if((dis[i] & 1) == ch) {cnt++;last = i;}}printf("%d\n", cnt);for(int i = 1; i <= n; i++) {if((dis[i] & 1) == ch) {printf("%d", i);if(i == last) {printf("\n");break;} else {printf(" ");}}}}}
}