Codeforces - 1176E - Cover it! - bfs

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

久了不寫bfs了。一開始用dfs寫，的確用dfs是頗有問題的，一些奇怪的狀況就會致使多染一些色。

注意無向圖的邊要開雙倍。

#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++;
    else
        cnteven++;
    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 Yinku
    freopen("Yinku.in", "r", stdin);
    //freopen("Yinku.out", "w", stdout);
#endif // Yinku
    int 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(" ");
                    }
                }
            }
        }
    }
}