write insert public function

[andrealorenzon]

Accepts 2 arguments: a key and a value

If we choose self-balancing trees, the insert function should automatically balance the tree (in that case, there's no need to implement a balance function). See link below.

Returns (2 options): 1) 0 if successfully insterted, 1 otherwise 2) maybe better: a shared_ptr to the new node if successfully inserted, nullptr otherwise

[andrealorenzon]

https://github.com/andrealorenzon/AP_exam/issues/13#issuecomment-449586743

[andrealorenzon]

draft:

       /**
       * @brief Inserts a new node into the binary search tree
       * @param key The key for the new node
       * @return The the inserted node otherwise nullptr
       */
      template <typedef K, typedef T>
      const std::shared_ptr<node<K,T> > insert(const K key) {
        std::shared_ptr<node <K,T> > current = root;
        std::shared_ptr<node <K,T> > parent = nullptr;
        while(current!=nullptr) {
          parent = current;
          if (key > current->key) {
            current = current->right;
          } else if (key < current->key) {
            current = current->left;
          } else {
            return nullptr;
          }
        }
        current = std::make_shared<node <key_t> >(key);
        current->parent = parent;
        if(parent == nullptr) {
          root = current;
        } else if (current->key > parent->key) {
          parent->right = current;
        } else if (current->key < parent->key) {
          parent->left = current;
        }
        return current;
      }