Language: C++

//
// author  : Abhishek
//
#include <bits/stdc++.h>
#include<chrono>

#define ll long long
#define db double
#define deb(x) cout<<#x<<": "<<x<<"\n"
#define all(x) x.begin(),x.end()
#define fo(i, n) for(i=0 ; i<n ; i++)
#define Fo(i, k, n) for(i=k ; i<n ; i++)

using namespace std;
using namespace chrono;

const ll mod = 1000000007;
const ll N = 300005;  // array size
const ll inf = 10000000000000000;


void print(vector<ll> v) {
    ll i;
    fo(i, v.size())cout << v[i] << " ";
    cout << "\n";
}

vector<ll> read(ll n) {
    ll i;
    vector<ll> arr(n, 0);
    fo(i, n)cin >> arr[i];
    return arr;
}

vector<ll> read(ll n, ll k) {
    ll i;
    vector<ll> arr(n + k, 0);
    Fo(i, k, n + k)cin >> arr[i];
    return arr;
}

bool isEven(ll n) {
    return n % 2 == 0;
}

bool isOdd(ll n) {
    return n % 2 != 0;
}


void solve() {
    ll t;
    cin >> (t);
    while (t--) {

        ll i, j, k, n, m, p, q, x, y, a, b, l, r;


    }
}

int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif

    auto start = high_resolution_clock::now();

    ll i, j, k, n, m, p, q, x, y, a, b, l, r;

    cin>>n;
    string s1,s2;
    cin>>s1>>s2;
    fo(i,n){
        cout<<s1[i]<<s2[i];
    }


    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<microseconds>(stop - start);
#ifndef ONLINE_JUDGE
    cerr << "Time: " << duration.count() / 1000.0 << endl;
#endif


    return 0;
}


//
// author  : Abhishek
//
#include <bits/stdc++.h>
#include<chrono>

#define ll long long
#define db double
#define deb(x) cout<<#x<<": "<<x<<"\n"
#define all(x) x.begin(),x.end()
#define fo(i, n) for(i=0 ; i<n ; i++)
#define Fo(i, k, n) for(i=k ; i<n ; i++)

using namespace std;
using namespace chrono;

const ll mod = 1000000007;
const ll N = 300005;  // array size
const ll inf = 10000000000000000;


void print(vector<ll> v) {
    ll i;
    fo(i, v.size())cout << v[i] << " ";
    cout << "\n";
}

vector<ll> read(ll n) {
    ll i;
    vector<ll> arr(n, 0);
    fo(i, n)cin >> arr[i];
    return arr;
}

vector<ll> read(ll n, ll k) {
    ll i;
    vector<ll> arr(n + k, 0);
    Fo(i, k, n + k)cin >> arr[i];
    return arr;
}

bool isEven(ll n) {
    return n % 2 == 0;
}

bool isOdd(ll n) {
    return n % 2 != 0;
}


void solve() {
    ll t;
    cin >> (t);
    while (t--) {

        ll i, j, k, n, m, p, q, x, y, a, b, l, r;


    }
}

int main() {

    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

#ifndef ONLINE_JUDGE
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif

    auto start = high_resolution_clock::now();

    ll i, j, k, n, m, p, q, x, y, a, b, l, r;

    cin>>n;
    string s1,s2;
    cin>>s1>>s2;
    fo(i,n){
        cout<<s1[i]<<s2[i];
    }


    auto stop = high_resolution_clock::now();
    auto duration = duration_cast<microseconds>(stop - start);
#ifndef ONLINE_JUDGE
    cerr << "Time: " << duration.count() / 1000.0 << endl;
#endif


    return 0;
}




Compiler output:
a.cc:112:10: error: redefinition of 'const long long int mod'
  112 | const ll mod = 1000000007;
      |          ^~~
a.cc:17:10: note: 'const long long int mod' previously defined here
   17 | const ll mod = 1000000007;
      |          ^~~
a.cc:113:10: error: redefinition of 'const long long int N'
  113 | const ll N = 300005;  // array size
      |          ^
a.cc:18:10: note: 'const long long int N' previously defined here
   18 | const ll N = 300005;  // array size
      |          ^
a.cc:114:10: error: redefinition of 'const long long int inf'
  114 | const ll inf = 10000000000000000;
      |          ^~~
a.cc:19:10: note: 'const long long int inf' previously defined here
   19 | const ll inf = 10000000000000000;
      |          ^~~
a.cc:117:6: error: redefinition of 'void print(std::vector<long long int>)'
  117 | void print(vector<ll> v) {
      |      ^~~~~
a.cc:22:6: note: 'void print(std::vector<long long int>)' previously defined here
   22 | void print(vector<ll> v) {
      |      ^~~~~
a.cc:123:12: error: redefinition of 'std::vector<long long int> read(long long int)'
  123 | vector<ll> read(ll n) {
      |            ^~~~
a.cc:28:12: note: 'std::vector<long long int> read(long long int)' previously defined here
   28 | vector<ll> read(ll n) {
      |            ^~~~
a.cc:130:12: error: redefinition of 'std::vector<long long int> read(long long int, long long int)'
  130 | vector<ll> read(ll n, ll k) {
      |            ^~~~
a.cc:35:12: note: 'std::vector<long long int> read(long long int, long long int)' previously defined here
   35 | vector<ll> read(ll n, ll k) {
      |            ^~~~
a.cc:137:6: error: redefinition of 'bool isEven(long long int)'
  137 | bool isEven(ll n) {
      |      ^~~~~~
a.cc:42:6: note: 'bool isEven(long long int)' previously defined here
   42 | bool isEven(ll n) {
      |      ^~~~~~
a.cc:141:6: error: redefinition of 'bool isOdd(long long int)'
  141 | bool isOdd(ll n) {
      |      ^~~~~
a.cc:46:6: note: 'bool isOdd(long long int)' previously defined here
   46 | bool isOdd(ll n) {
      |      ^~~~~
a.cc:146:6: error: redefinition of 'void solve()'
  146 | void solve() {
      |      ^~~~~
a.cc:51:6: note: 'void solve()' previously defined here
   51 | void solve() {
      |      ^~~~~
a.cc:157:5: error: redefinition of 'int main()'
  157 | int main() {
      |     ^~~~
a.cc:62:5: note: 'int main()' previously defined here
   62 | int main() {
      |     ^~~~