RT,CSES-1756
我用的 DFS 树的解法，但不知道为什么会挂掉，求调

#include <cstdio>
#include <cmath>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>
#include <vector>
#include <map>
#include <unordered_map>
#include <set>
#define ll long long
#define fp(a,b,c) for(ll a=b;a<=c;a++)
#define fd(a,b,c) for(ll a=b;a>=c;a--)
#define pii pair<int,int>
#define pll pair<ll,ll>
#define fr first
#define sd second
#define mod 1000000007
#define inf 0x3f3f3f3f

using namespace std;


inline int rd(){
	int x = 0, f = 1;char ch = getchar();
	while(ch < '0' || ch > '9'){if(ch == '-')f = -1;ch = getchar();}
	while(ch >= '0' && ch <= '9')x = (x<<1) + (x<<3) + (ch^48),ch = getchar();
	return x * f;}
inline ll lrd(){
	ll x = 0, f = 1;char ch = getchar();
	while(ch < '0' || ch > '9'){if(ch == '-')f = -1;ch = getchar();}
	while(ch >= '0' && ch <= '9')x = (x<<1) + (x<<3) + (ch^48),ch = getchar();
	return x * f;}

const int maxN=2*1e5+10;
int n,m;
vector<int>g[maxN];
int dep[maxN],fa[maxN];

inline void dfs(int now,int f){
	fa[now]=f,dep[now]=dep[f]+1;
	for(int x:g[now]){
		if(x==f)continue;
		if(!dep[x])dfs(x,now),cout << now << " " << x << endl;
		else if(dep[x]<=dep[now]){
			cout << x << ' ' << now << endl;
		}
	}
		
}

signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0),cout.tie(0); 
	n=rd(),m=rd();
	fp(i,1,m){
		int u=rd(),v=rd();
		g[u].push_back(v),g[v].push_back(u);
	}
	fp(i,1,n)
		if(!dep[i]) dfs(i,i);
	
	
	
	return 0;
}