C++ linked list example (base on Arduino environment)

linked list example

/*
  C++ linked list example (base on Arduino environment)
  by Dhmadhawach Horesuwan
  1/7/2560 , Bangkok , Thailand
*/
class linked_list {
    struct node {
      char data;// you can change this to datatype which you want.
      node* next;
      node* previous;
    };
  public:
    unsigned int _length = 0;
    node* head = NULL;
    node* tail = NULL;

    void add(char data_add) {
      node* new_node = new node();
      new_node->data = data_add;
      if (_length == 0) {
        head = new_node;
        tail = new_node;
      }
      else {
        tail->next = new_node;
        new_node -> previous = tail;
        tail = new_node;
      }
      _length++;
    }
    char remove() {
      if (_length > 0) {
        char end_data = tail -> data;
        node* end = tail;
        tail = tail -> previous;
        delete end;
        _length--;
        return end_data;
      }
      return 0;
    }
    char pop() {
      if (_length > 0) {
        char head_data = head -> data;
        node* first = head;
        head = head -> next;
        delete first;
        _length--;
        return head_data;
      }
      return 0;
    }
    void insert(unsigned int index, char data_add) {
      node* new_node = new node();
      new_node->data = data_add;
      if (index >= _length) {
        add(data_add);
      }
      else if(index == 0){
        new_node->next = head;
        head->previous = new_node;
        head = new_node;
        _length++;
      }
      
      else {
        node* crr = head;
        for (int i = 0; i < index; i++) {
          crr = crr->next;
        }
        
        new_node->previous = crr->previous;
        new_node->next = crr;
        crr->previous->next = new_node;
        crr->previous = new_node;
        _length++;
      }
    }
    void print() {
      node* crr = head;
      for (int i = 0; i < _length; i++) {
        Serial.println(crr->data);
        crr = crr->next;
      }
    }
};

int freeRam () {
  extern int __heap_start, *__brkval;
  int v;
  return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
}

void setup() {
  
  Serial.begin(115200);
  Serial.println("available RAM before add = " + String(freeRam()));
  linked_list simple;
  /*
  for (char i = 'A'; i <= 'Z'; i++) {
    simple.add(i);
  }
  Serial.println("data length = " + String(simple._length));
  Serial.println("available RAM after add= " + String(freeRam()));
  simple.print();
  for (char i = 'A'; i <= 'Z'; i++) {
    Serial.print(simple.remove()); // print which that remove
    //Serial.print(simple.pop());
  }
  Serial.println();

  Serial.println("available RAM after delete all = " + String(freeRam()));
  Serial.println("data length = " + String(simple._length));
  */
  for (char i = '0'; i <= '9'; i++) {
    simple.add(i);
  }
  simple.insert(10,'X');
  simple.print();
}

void loop() {
  // put your main code here, to run repeatedly:

}