图的根节点-数据结构作业。。

#include <iostream>
#include <queue>
using namespace std;

const int maxn = 100000;
const int maxm = 300000;
struct Graph {
int NE , NV;
int head[maxn];
struct Node{
int next , v;
Node(){}
Node(int a,int b):v(a) , next(b){}
}E[maxm];
void clear(int n) {
NV = n;
NE = 0;
memset(head,-1,sizeof(int)*n);
}
inline void Insert(int u,int v) {
E[NE] = Node(v , head[u]);
head[u] = NE ++;
}
void Print() {
for(int i = 0 ; i < NV ; i ++) {
printf("%d:",i);
for(int j = head[i]; j != -1; j = E[j].next){
printf(" %d",E[j].v);
}
puts("");
}
}
}G;

int n, m, ctr;
int visit[maxn];

void BFS(int cur) {
queue<int> Q;
Q.push(cur);
visit[cur] = 1;
while( !Q.empty() ){
for(int i = G.head[Q.front()]; i != -1; i = G.E[i].next ){
if(!visit[G.E[i].v]){
visit[G.E[i].v] = 1;
Q.push(G.E[i].v);
ctr++;
if(ctr >= n - 1) printf("%d为根节点\n", cur);
}
}
Q.pop();
}
}

int main(){
cin >> n;//读入顶点数
G.clear(n);
cin >> m;//读入边数
int begin, end;
for(int i = 0; i < m; ++i) {
scanf("%d %d", &begin, &end);
G.Insert(begin, end);
}
//遍历图 输出根节点
for(int i = 0; i < n; ++i){
memset(visit, 0, sizeof(visit));
ctr = 0;
BFS(i);
}
return 0;
}