Graph. Find Bridge (105)

#include <vector>
#include <iostream>
#include <algorithm>
#include <fstream>
#include <iomanip>
#include <stack>
#include <map>
#include <set>
#include <string>
#include <queue>

using namespace std;
typedef long long ll;

int n, m;
vector<vector<int> > g(n);
vector<bool> used;
vector<int> tin, fup;
vector<pair<int, int> > ans;
int timer;

void dfs(int v, int p = -1)
{
    used[v] = true;
    tin[v] = fup[v] = timer++;
    for (int i = 0; i < g[v].size(); i++)
    {
        int to = g[v][i];
        if (to == p)
        {
            continue;
        }

        if (used[to])
        {
            fup[v] = min(fup[v], tin[to]);
        }
        else
        {
            dfs(to, v);
            fup[v] = min(fup[v], fup[to]);
            if (fup[to] > tin[v])
            {
                ans.push_back(make_pair(v+1, to+1));
            }
        }
    }
}

void find_bridges()
{
    timer = 0;
    for (int i = 0; i < n; i++)
    {
        if (!used[i])
        {
            dfs(i);
        }
    }
}
int main()
{
    cin >> n >> m;
    g.resize(n);
    used.resize(n);
    tin.resize(n);
    fup.resize(n);
    vector<pair<int, int> > Ans_pair;
    for (int i(0); i < m; i++)
    {
        int x, y;
        cin >> x >> y;
        x--; y--;
        g[x].push_back(y);
        g[y].push_back(x);
        Ans_pair.push_back(make_pair(x+1, y+1));
    }
    find_bridges();
    cout << ans.size() << endl;
    for (int i(0); i < Ans_pair.size(); i++)
    {
        for (int j(0); j < ans.size(); j++)
        {
            if (ans[j] == Ans_pair[i] || (ans[j].first == Ans_pair[i].second && ans[j].second == Ans_pair[i].first))
            {
                cout << i+1 << endl;
                break;
            }
        }
    }
    return 0;
}