Check Completeness of a Binary Tree in C++

Suppose we have a binary tree. We have to check whether the tree is a complete binary tree or not. A complete binary tree of level n, has n-1 complete levels, and all nodes at level n, are filled from the left. So if the input tree is like −

Then the output will be true, As this is complete binary tree.

To solve this, we will follow these steps −

• If tree is empty, then return null

• make a queue q and insert root into it

• set flag := true

• while q has some elements

  • sz := size of the queue

  • while sz is not 0

    • node := node after deleting from queue

    • if node has left subtree, then

      • if flag is set, then insert left subtree of node into the q, othereise return false

    • otherwise flag := false

    • if node has right subtree, then

      • if flag is set, then insert right subtree of node into the q, otherwise return false

    • flag := false

    • sz := sz – 1

• return ture

Let us see the following implementation to get better understanding −

Example

 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;
}
class Solution {
   public:
   bool isCompleteTree(TreeNode* root) {
      if(!root)return true;
      queue <TreeNode*> q;
      q.push(root);
      bool isComplete = true;
      while(!q.empty()){
         int sz = q.size();
         while(sz--){
            TreeNode* node = q.front();
            q.pop();
            if(node->left){
               if(isComplete){
                  q.push(node->left);
               }else return false;
            }else{
               isComplete = false;
            }
            if(node->right){
               if(isComplete){
                  q.push(node->right);
               }else return false;
            }else{
               isComplete = false;
            }
         }  
      }
      return true;
   }
};
main(){
   vector<int> v = {1,2,3,4,5,6};
   TreeNode *r1 = make_tree(v);
   Solution ob;
   cout << (ob.isCompleteTree(r1));
}

Input

{1,2,3,4,5,6}

Output

1