Fragile network

input/code.cpp: In function 'int main()':
input/code.cpp:71:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   71 |         for(int j=1; j<leaves[leaf_list[i]].size(); j++)
      |                      ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
input/code.cpp: In function 'void Test()':
input/code.cpp:28:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   28 |     freopen("temp\\in.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:77:32: warning: 's' may be used uninitialized in this function [-Wmaybe-uninitialized]
   77 |         ans.push_back(make_pair(s, hold_list[num_holdleaves-1]));
      |                       ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=100005;

vector<int> edges[N];
vector<int> leaves[N];

bool DFS(int father, int u)
{
    bool son_flag = true;
    for(auto v: edges[u])
    {
        if(v == father)
            continue;
        if(DFS(u, v))
        {
            // v is a leave
            leaves[u].push_back(v);
        }
        son_flag = false;
    }
    return son_flag;
}

void Test()
{
    freopen("temp\\in.txt", "r", stdin);
}
int main()
{
    // Test();
    int n;
    cin >> n;
    for(int i=1; i<=n-1; i++)
    {
        int a, b;
        cin >> a >> b;
        edges[a].push_back(b);
        edges[b].push_back(a);
    }

    int s;
    for(int i=1; i<=n; i++)
        if(edges[i].size() > 1) {
            s = i;
            break;
        }
    DFS(0, s);

    vector<int> leaf_list, hold_list;
    vector<pair<int, int> > ans;
    for(int i=1; i<=n; i++)
    {
        if(leaves[i].empty())
            continue;
        leaf_list.push_back(i);
    }
    int num_subtrees = leaf_list.size();
    if(num_subtrees % 2)
    {
        // A single sub tree can't form
        int tail = leaf_list[num_subtrees-1];
        ans.push_back(make_pair(s, leaves[tail][0]));
    } 
    // Make all first node connected, then to the hold_list
    for(int i=0; i<num_subtrees-1; i+=2)
        ans.push_back(make_pair(leaves[leaf_list[i]][0], 
                                leaves[leaf_list[i+1]][0]));
    for(int i=0; i<num_subtrees; i++)
        for(int j=1; j<leaves[leaf_list[i]].size(); j++)
            hold_list.push_back(leaves[leaf_list[i]][j]);


    int num_holdleaves = hold_list.size();
    if(num_holdleaves % 2)
        ans.push_back(make_pair(s, hold_list[num_holdleaves-1]));
    for(int i=0; i<num_holdleaves - 1; i+=2)
        ans.push_back(make_pair(hold_list[i], hold_list[i+1]));

    cout << ans.size() << endl;
    for(auto p: ans)
        cout << p.first << " " << p.second <<endl;

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: WRONG ANSWER

Test 5

Verdict: WRONG ANSWER

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: WRONG ANSWER