Complete Binary Tree Inserter in C++

As we know a complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible. We have to write a data structure CBTInserter that is initialized with a complete binary tree and it supports the following operations−

• CBTInserter(TreeNode root) this initializes the data structure on a given tree with head node root;

• CBTInserter.insert(int v) will used to insert a TreeNode into the tree with a value node.val = v so that the tree remains complete, and returns the value of the parent of the inserted TreeNode;

• CBTInserter.get_root() this will return the head node of the tree.

So for example, if we initialize the tree as [1,2,3,4,5,6], then insert 7 and 8, then try to get the tree, the output will be: 3, 4,[1,2,3,4,5,6,7,8], the 3 is because 7 will be inserted under 3, and 4 is because 8 will be inserted under 4.

To solve this, we will follow these steps −

• Define a queue q, and a root

• The initializer will take the full binary tree, then work as follows

• set root as given root, insert root into the q

• while true −

  • if left of root is present, then insert left of root into q, otherwise break the loop

  • if right of root is present, then insert right of root into q and delete front node from q, otherwise break the loop

• From the insert method, it will take the value v

• set parent := front element of q, temp := new node with value v, and insert it into q

• if left of parent is not present, then set left of parent := temp otherwise delete front element from q, and insert temp as right child of parent

• return the value of parent

• from the getRoot() method, return the root

Example(C++)

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class TreeNode{
   public:
      int val;
      TreeNode *left, *right;
      TreeNode(int data){
         val = data;
         left = NULL;
         right = NULL;
      }
};
void insert(TreeNode **root, int val){
   queue<TreeNode*> q;
   q.push(*root);
   while(q.size()){
      TreeNode *temp = q.front();
      q.pop();
      if(!temp->left){
         if(val != NULL)
            temp->left = new TreeNode(val);
         else
            temp->left = new TreeNode(0);
         return;
      } else {
         q.push(temp->left);
      }
      if(!temp->right){
         if(val != NULL)
            temp->right = new TreeNode(val);
         else
            temp->right = new TreeNode(0);
         return;
      } else {
         q.push(temp->right);
      }
   }
}
TreeNode *make_tree(vector<int> v){
   TreeNode *root = new TreeNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      insert(&root, v[i]);
   }
   return root;
}
void tree_level_trav(TreeNode*root){
   if (root == NULL) return;
   cout << "[";
   queue<TreeNode *> q;
   TreeNode *curr;
   q.push(root);
   q.push(NULL);
   while (q.size() > 1) {
      curr = q.front();
      q.pop();
      if (curr == NULL){
         q.push(NULL);
      } else {
         if(curr->left)
            q.push(curr->left);
         if(curr->right)
            q.push(curr->right);
         if(curr == NULL || curr->val == 0){
            cout << "null" << ", ";
         } else{
            cout << curr->val << ", ";
         }
      }
   }
   cout << "]"<<endl;
}
class CBTInserter {
public:
   queue <TreeNode*> q;
   TreeNode* root;
   CBTInserter(TreeNode* root) {
      this->root = root;
      q.push(root);
      while(1){
         if(root->left){
            q.push(root->left);
         }
         else break;
         if(root->right){
            q.push(root->right);
            q.pop();
            root = q.front();
         }
         else break;
      }
   }
   int insert(int v) {
      TreeNode* parent = q.front();
      TreeNode* temp = new TreeNode(v);
      q.push(temp);
      if(!parent->left){
         parent->left = temp;
      } else {
         q.pop();
         parent->right = temp;
      }
      return parent->val;
   }
   TreeNode* get_root() {
      return root;
   }
};
main(){
   vector<int> v = {1,2,3,4,5,6};
   TreeNode *root = make_tree(v);
   CBTInserter ob(root);
   cout << (ob.insert(7)) << endl;
   cout << (ob.insert(8)) << endl;
   tree_level_trav(ob.get_root());
}

Input

Initialize the tree as [1,2,3,4,5,6], then insert 7 and 8 into the tree, then find root

Output

3
4
[1, 2, 3, 4, 5, 6, 7, 8, ]