This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2")
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define sz size
const int mxN = 1e5;
vector<int> adj[mxN];
int d[mxN];
int low[mxN];
void dfs(int c, int depth, int p){
d[c] = depth;
low[c] = d[c];
int op = p;
for (int i : adj[c]){
if (i == p){p = -1; continue;}
if (d[i] != -1){ low[c] = min(low[c],d[i]); continue;}
dfs(i,depth+1,c);
low[c] = min(low[c], low[i]);
}
if (low[c] >= depth && op != -1) cout << c+1 << " " << op+1 << '\n';
}
int p[mxN][2];
int ssize[mxN][2];
int gpar(int c, int w){
if (p[c][w] == c) return c;
p[c][w] = gpar(p[c][w],w);
return p[c][w];
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n,m;
cin >> n >> m;
for (int i = 0; i < n; i++) for (int j = 0; j < 2; j++) {p[i][j] = i; ssize[i][j] = 1;}
int a,b;
int x,y;
for (int i = 0; i < m; i++){
cin >> a >> b;
a -= 1;
b -= 1;
x = gpar(a,0);
y = gpar(b,0);
if (x != y){
adj[a].pb(b);
adj[b].pb(a);
if (ssize[x][0] < ssize[y][0]) swap(x,y);
ssize[x][0] += ssize[y][0];
p[y][0] = x;
continue;
}
x = gpar(a,1);
y = gpar(b,1);
if (x == y) continue;
if (ssize[x][1] < ssize[y][1]) swap(x,y);
ssize[x][1] += ssize[y][1];
p[y][1] = x;
adj[a].pb(b);
adj[b].pb(a);
}
for (int i = 0; i < n; i++) d[i] = -1;
for (int i = 0; i < n; i++) if (d[i] == -1) dfs(i,0,-1);
}
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