left_rotate.cpp

ostream& operator<<(ostream& os, const vector<int>& v){
    os << "[ ";
    for(int i = 0; i<=v.size()-1; i++){
        os << "'" << v.at(i) << "'";
        if(i != v.size()-1) os << ',';
    }
    os << " ]";
    return os;
}

vector<int> rotateA(vector<int> a, int d){
  int n = a.size();
  if( d == 0 || d == n) return a;
  
  d = d%n;

  if(n == 2*d){ 
    // special case when n = 2d, then we just want to swap(i, i+d)
    // to avoid an inf. loop
    for(int i = 0; i < n/2; i++){
      int tmp = a.at(i);
      a.at(i) = a.at(i+d);
      a.at(i+d) = tmp;
    }
  }
  else{ 
    //in all other cases, start from the end, put elem. into its destination, 
    // put elem. at dest into its new dest etc.
    int i = n-1;
    int cycle = n;
    int curr = a.at(i);
    while(cycle){
      int dest = i-d;
      if(dest < 0) dest = n+dest;
      int tmp = a.at(dest);
      a.at(dest) = curr;
      i = dest;
      curr = tmp;
      cycle--;
    }
  }
  return a;
}

int main(){
  vector<int> items = {1,2,3,4,5,6};
  cout << rotateA(items, 3) << endl;
  return 0;
}