LightOJ 1009 Back to Underworld

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#include <bits/stdc++.h>using namespace std;int fa[20005];vector<int> g[20005];int bfs(int start){    int cnt[2] = {0};    queue<int> q;    q.push(start); fa[start] = 0; cnt[0]++;    while(!q.empty())    {        int p = q.front(); q.pop();        for(int i = 0; i < g[p].size(); i++)        {            int k = g[p][i];            if(fa[k] == -1)                q.push(k), fa[k] = fa[p]^1, cnt[fa[k]]++;        }    }    return max(cnt[0], cnt[1]);}int main(){    int T;    scanf("%d", &T);    for(int ca = 1; ca <= T; ca++)    {        for(int i = 1; i <= 20000; i++)            g[i].clear(), fa[i] = 0;        int n, a, b;        scanf("%d", &n);        for(int i = 0; i < n; i++)        {            scanf("%d%d", &a, &b);            fa[a] = fa[b] = -1;            g[a].push_back(b);            g[b].push_back(a);        }        int ans = 0;        for(int i = 1; i <= 20000; i++)            if(fa[i] == -1) ans += bfs(i);        printf("Case %d: %d\n", ca, ans);    }    return 0;}