Connect cities

input/code.cpp: In function 'int main()':
input/code.cpp:55:14: warning: format '%d' expects argument of type 'int', but argument 2 has type 'std::set<int>::size_type' {aka 'long unsigned int'} [-Wformat=]
   55 |     printf("%d\n", ans.size()-1);
      |             ~^     ~~~~~~~~~~~~
      |              |               |
      |              int             std::set<int>::size_type {aka long unsigned int}
      |             %ld
input/code.cpp: In function 'void Test()':
input/code.cpp:33:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   33 |     freopen("temp\\in.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
input/code.cpp: In function 'int main()':
input/code.cpp:39:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   39 |     scanf("%d%d", &n, &m);
      |     ~~~~~^~~~~~~~~~~~...

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<set>
typedef long long LL;

const int N=10005;

// Union&Find Start
int father[N], size[N];

int Find(int x)
{
    if(father[x]==x)
        return x;
    return father[x]=Find(father[x]);
}
void Union(int x, int y)
{
    x = Find(x), y=Find(y);
    if(x==y)
        return;
    if(size[x] < size[y])
        std::swap(x, y);
    father[y] = x;
    size[x] += size[y];
}
// Union&Find End


void Test()
{
    freopen("temp\\in.txt", "r", stdin);
}
int main()
{
    // Test();
    int n, m;
    scanf("%d%d", &n, &m);

    for(int i=1; i<=n; i++)
        father[i]=i;
    for(int i=1; i<=m; i++)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        Union(a, b);
    }

    std::set<int> ans;
    for(int i=1; i<=n; i++)
        ans.insert(Find(i));

    // P
    printf("%d\n", ans.size()-1);
    int last = *ans.begin();
    for(auto it=ans.begin(); it!=ans.end(); it++)
    {
        if(it==ans.begin())
            continue;
        int now = *it;
        printf("%d %d\n", last, now);
        last = now;
    }

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: RUNTIME ERROR

Test 7

Verdict: RUNTIME ERROR

Test 8

Verdict: RUNTIME ERROR

Test 9

Verdict: RUNTIME ERROR

Test 10

Verdict: RUNTIME ERROR

Test 11

Verdict: RUNTIME ERROR

Test 12

Verdict: ACCEPTED