Problem p03760 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define fo(a, b) for (int a = 0; a < b; a++)
#define Sort(a) sort(a.begin(), a.end())
#define rev(a) reverse(a.begin(), a.end())
#define fi first
#define se second
#define co(a) cout << a << endl
#define sz size()
#define bgn begin()
#define en end()
#define pb(a) push_back(a)
#define pp() pop_back()
#define V vector
#define P pair
#define V2(a, b, c) V<V<int>> a(b, V<int>(c))
#define V2a(a, b, c, d) V<V<int>> a(b, V<int>(c, d))
#define incin(a)                                                               \
  int a;                                                                       \
  cin >> a
#define yuko(a) setprecision(a)
#define uni(a) a.erase(unique(a.begin(), a.end()), a.end())
#define pri priority_queue
// #define min min<int>
// #define max max<int>
template <class T> void cou(vector<vector<T>> a) {
  int b = a.size();
  int c = a[0].size();
  fo(i, b) {
    fo(j, c) {
      cout << a[i][j];
      if (j == c - 1)
        cout << endl;
      else
        cout << ' ';
    }
  }
}
/*template<>
 void cou(vector<vector<char>> a){
  int b=a.size();
  int c=a[0].size();
  fo(i,b){
    fo(j,c){
      cout<<a[i][j];
      if(j==c-1)
        cout<<endl;
      else
        cout<<' ';
    }
  }
}*/
int wari(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
int keta(int a) {
  double b = a;
  b = log10(b);
  int c = b;
  return c + 1;
}
int souwa(int a) { return a * (a + 1) / 2; }
/*int lcm(int a,int b){
 int d=a,e=b,f;
 if(a<b)
   swap(a,b);
 int c,m=1;
 while(m){
   c=a%b;
   if(c==0){
      f=b;
     m--;
   }
   else{
     a=b;
     b=c;
}
 }
 return d*e/f;
   }
*/
int gcm(int a, int b) {
  int d = a, e = b, f;
  if (a < b)
    swap(a, b);
  int c, m = 1;
  while (m) {
    c = a % b;
    if (c == 0) {
      f = b;
      m--;
    } else {
      a = b;
      b = c;
    }
  }
  return f;
} /*
/*bool prime(int a){
  if(a<2)
    return false;
  else if(a==2)
    return true;
  else if(a%2==0)
    return false;
  double b=sqrt(a);
   for(int i=3;i<=b;i+=2){
     if(a%i==0){
       return false;
     }
   }
  return true;
}*/
struct Union {
  vector<int> par;
  Union(int a) { par = vector<int>(a, -1); }
  int find(int a) {
    if (par[a] < 0)
      return a;
    else
      return par[a] = find(par[a]);
  }
  bool same(int a, int b) { return find(a) == find(b); }
  int Size(int a) { return -par[find(a)]; }
  void unite(int a, int b) {
    a = find(a);
    b = find(b);
    if (a == b)
      return;
    if (Size(b) > Size(a))
      swap<int>(a, b);
    par[a] += par[b];
    par[b] = a;
  }
};
int ketas(int a) {
  string b = to_string(a);
  int c = 0;
  fo(i, keta(a)) { c += b[i] - '0'; }
  return c;
}
/*int gcm(int a,int b){
    if(b==0)
        return a;
    return gcm(b,a%b);
}*/
int lcm(int a, int b) { return a / gcm(a, b) * b; }
/*struct aa{
        vector<int> gt;
  aa(int n){
        gt= vector<int>(n, 1);
  }
        void c(V<int> d,int b){
        if(d[b]==0){
                gt[d[b]-1]++;
                gt[gt.sz-1]++;
        }
        else{
                gt[d[b]-1]++;
                c(d,d[d[b]]-1);
        }
}
void cok(int a){
        cout<<gt[a-1]<<endl;
        fo(i,a-1)
        cout<<gt[i]<<endl;
}
};
*/
/*struct dfs(){

}*/
bool fe(int a, int b) {
  a %= 10;
  b %= 10;
  if (a == 0)
    a = 10;
  if (b == 0)
    b = 10;
  if (a > b)
    return true;
  else
    return false;
}
int INF = 1000000007;
struct edge {
  int s, t, d;
};
signed main() {
  string a, b, c;
  int i;
  cin >> a >> b;
  for (i = 0; i < b.size(); i++) {
    c.pb(a.at(i));
    c.pb(b.at(i));
  }
  if (a.size() != b.size())
    c.pb(a.at(i + 1));
  cout << c << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define fo(a, b) for (int a = 0; a < b; a++)
#define Sort(a) sort(a.begin(), a.end())
#define rev(a) reverse(a.begin(), a.end())
#define fi first
#define se second
#define co(a) cout << a << endl
#define sz size()
#define bgn begin()
#define en end()
#define pb(a) push_back(a)
#define pp() pop_back()
#define V vector
#define P pair
#define V2(a, b, c) V<V<int>> a(b, V<int>(c))
#define V2a(a, b, c, d) V<V<int>> a(b, V<int>(c, d))
#define incin(a)                                                               \
  int a;                                                                       \
  cin >> a
#define yuko(a) setprecision(a)
#define uni(a) a.erase(unique(a.begin(), a.end()), a.end())
#define pri priority_queue
// #define min min<int>
// #define max max<int>
template <class T> void cou(vector<vector<T>> a) {
  int b = a.size();
  int c = a[0].size();
  fo(i, b) {
    fo(j, c) {
      cout << a[i][j];
      if (j == c - 1)
        cout << endl;
      else
        cout << ' ';
    }
  }
}
/*template<>
 void cou(vector<vector<char>> a){
  int b=a.size();
  int c=a[0].size();
  fo(i,b){
    fo(j,c){
      cout<<a[i][j];
      if(j==c-1)
        cout<<endl;
      else
        cout<<' ';
    }
  }
}*/
int wari(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
int keta(int a) {
  double b = a;
  b = log10(b);
  int c = b;
  return c + 1;
}
int souwa(int a) { return a * (a + 1) / 2; }
/*int lcm(int a,int b){
 int d=a,e=b,f;
 if(a<b)
   swap(a,b);
 int c,m=1;
 while(m){
   c=a%b;
   if(c==0){
      f=b;
     m--;
   }
   else{
     a=b;
     b=c;
}
 }
 return d*e/f;
   }
*/
int gcm(int a, int b) {
  int d = a, e = b, f;
  if (a < b)
    swap(a, b);
  int c, m = 1;
  while (m) {
    c = a % b;
    if (c == 0) {
      f = b;
      m--;
    } else {
      a = b;
      b = c;
    }
  }
  return f;
} /*
/*bool prime(int a){
  if(a<2)
    return false;
  else if(a==2)
    return true;
  else if(a%2==0)
    return false;
  double b=sqrt(a);
   for(int i=3;i<=b;i+=2){
     if(a%i==0){
       return false;
     }
   }
  return true;
}*/
struct Union {
  vector<int> par;
  Union(int a) { par = vector<int>(a, -1); }
  int find(int a) {
    if (par[a] < 0)
      return a;
    else
      return par[a] = find(par[a]);
  }
  bool same(int a, int b) { return find(a) == find(b); }
  int Size(int a) { return -par[find(a)]; }
  void unite(int a, int b) {
    a = find(a);
    b = find(b);
    if (a == b)
      return;
    if (Size(b) > Size(a))
      swap<int>(a, b);
    par[a] += par[b];
    par[b] = a;
  }
};
int ketas(int a) {
  string b = to_string(a);
  int c = 0;
  fo(i, keta(a)) { c += b[i] - '0'; }
  return c;
}
/*int gcm(int a,int b){
    if(b==0)
        return a;
    return gcm(b,a%b);
}*/
int lcm(int a, int b) { return a / gcm(a, b) * b; }
/*struct aa{
        vector<int> gt;
  aa(int n){
        gt= vector<int>(n, 1);
  }
        void c(V<int> d,int b){
        if(d[b]==0){
                gt[d[b]-1]++;
                gt[gt.sz-1]++;
        }
        else{
                gt[d[b]-1]++;
                c(d,d[d[b]]-1);
        }
}
void cok(int a){
        cout<<gt[a-1]<<endl;
        fo(i,a-1)
        cout<<gt[i]<<endl;
}
};
*/
/*struct dfs(){

}*/
bool fe(int a, int b) {
  a %= 10;
  b %= 10;
  if (a == 0)
    a = 10;
  if (b == 0)
    b = 10;
  if (a > b)
    return true;
  else
    return false;
}
int INF = 1000000007;
struct edge {
  int s, t, d;
};
signed main() {
  string a, b, c;
  int i;
  cin >> a >> b;
  for (i = 0; i < b.size(); i++) {
    c.pb(a.at(i));
    c.pb(b.at(i));
  }
  if (a.size() != b.size())
    c.pb(a.at(a.size() - 1));
  cout << c << endl;
}