poj3310Caterpillar(树直径)

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1.连通性
2.树
3.求直径
struct UNION {    int F[101];    void init(int n = 0) {        memset(F, -1,sizeof F);    }    int find(int u) {        return F[u] == -1?u:F[u] = find(F[u]);    }    bool unit(int u,int v) {        int t1 = find(u);        int t2 = find(v);        if (t1 == t2) return false;        F[t1] = t2;        return true;    }    bool check(int n) {        int cnt = 0;        for (int i = 1;i <= n;++i)            if (F[i] == -1) cnt++;        return cnt == 1;    }}UF;vector<vector<int> > G;int dep[123];int pre[123];void dfs(int u,int fa) {    for (int i = 0;i < G[u].size();++i) {        int v = G[u][i];        if (v == fa) continue;        dep[v] = dep[u] + 1;        pre[v] = u;        dfs(v, u);    }}bool mark[123];int main(int argc, const char * argv[]){        // freopen("in.txt","r",stdin);    // freopen("out.txt","w",stdout);    // clock_t _ = clock();    int n, m;    while(~scanf("%d", &n) && n) {        scanf("%d", &m);        UF.init();        int u, v;        G.clear();        G.resize(n + 2);        bool flag = true;        for (int i = 1;i <= m;++i) {            scanf("%d%d", &u, &v);            G[u].push_back(v);            G[v].push_back(u);            if (!UF.unit(u, v)) flag = false;        }        if (!flag || !UF.check(n)) {            printf("Graph %d is not a caterpillar.\n", ++nCase);            continue;        }        int st, ed;        st = 1;        dep[1] = 0;        dfs(st, -1);        ed = 1;        for (int i = 1;i <= n;++i)            if (dep[i] > dep[ed]) ed = i;        dep[st = ed] = 0;        dfs(st, -1);        ed = 1;        for (int i = 1;i <=n;++i)            if (dep[i] > dep[ed]) ed = i;        memset(mark, false,sizeof mark);        int o = ed;        while(o != st) {            mark[o] = true;            o = pre[o];        }        mark[st] = true;        flag = true;        for (int i = 1;i <= n;++i) {            if (mark[i]) continue;            bool tmp = false;            for (int j = 0;j < G[i].size();++j) {                if (mark[G[i][j]]) {                    tmp = true;                    break;                }            }            if (!tmp) flag = false;        }        if (!flag) printf("Graph %d is not a caterpillar.\n", ++nCase);        else printf("Graph %d is a caterpillar.\n", ++nCase);    }    // printf("\nTime cost: %.2fs\n", 1.0 * (clock() - _) / CLOCKS_PER_SEC);    return 0;}
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