poj3310Caterpillar(树直径)

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;
}