We know **tree** is a non-linear data structure. It has no limitation on the number of child. A binary tree has a limitation as any node of the tree has atmost two children: a left and a right child.

__Introduction to Complete Binary Tree:__

A binary tree is said to be a complete binary tree when all the levels are filled completely except the lowest level nodes which are filled from as left as possible.

__Some terminology:__

**Root**– Node in which no edge is coming from the parent. Example -node A**Child**– Node having some incoming edge is called child. Example – nodes B, H are the child of A and D respectively.**Sibling**– Nodes having the same parent are sibling. Example- J, K are siblings as they have the same parent E.**Degree of a node**– Number of children of a particular parent. Example- Degree of A is 2 and Degree of H is 1. Degree of L is 0.**Internal/External nodes**– Leaf nodes are external nodes and non leaf nodes are internal nodes.**Level**– Count nodes in a path to reach a destination node. Example- Level of node H is 3 as nodes A, D and H themselves form the path.**Height**– Number of edges to reach the destination node, Root is at height 0. Example – Height of node E is 2 as it has two edges from the root.

__Properties of Complete Binary Tree:__

- A complete binary tree is said to be a proper binary tree where all leaves have the same depth.
- In a complete binary tree number of nodes at depth
**d**is**2**.^{d} - In a complete binary tree with
**n**nodes height of the tree is**log(n+1)**. - All the levels
**except the last level**are completely full.

__Perfect Binary Tree____ vs Complete Binary Tree:__

A binary tree of height ‘h’ having the maximum number of nodes is a **perfect** binary tree.

For a given height **h**the maximum number of nodes is **2 ^{h+1}-1**.

A **complete** binary tree of height h is a proper binary tree up to height **h-1**and in the last level element are stored in left to right order.

**Example 1:**

The height of the given binary tree is 2 and the maximum number of nodes in that tree is n=2^{h+1}-1 = 2^{2+1}-1 = 2^{3}-1 = **7**.

Hence we can conclude it is **a perfect binary tree**.

Now for a complete binary tree, It is full up to height **h-1** ie; 1, and the last level elements are stored in left to right order. Hence it is a complete Binary tree also. Here is the representation of elements when stored in an array

In the array, all the elements are stored continuously.

**Example 2:**

Height of the given binary tree is 2 and the maximum number of nodes that should be there are 2^{h+1 }– 1 = 2^{2+1 }– 1 = 2^{3 }– 1 = **7**.

But the number of nodes in the tree is **6**. Hence it is **not a perfect binary tree**.

Now for a complete binary tree, It is full up to height **h-1** ie; **1**, and the last level element are stored in left to right order. Hence this is a **complete binary tree**. Store the element in an array and it will be like;

**Example 3:**

The height of the binary tree is 2 and the maximum number of nodes that can be there is 7, but there are only 5 nodes hence it is **not a perfect binary tree**.

In case of a complete binary tree, we see that in the last level elements are not filled from left to right order. So it is **not a complete binary tree**.

The elements in the array are not continuous.

__Full Binary Tree vs Complete Binary tree:__

For a full binary tree, every node has either 2 children or 0 children.

**Example 1:**

In the given binary tree there is no node having degree 1, either 2 or 0 children for every node, hence it is **a full binary tree**.

For a complete binary tree, elements are stored in level by level and not from the leftmost side in the last level. Hence this is **not a complete binary tree**. The array representation is:

**Example 2:**

In the given binary tree there is no node having degree 1. Every node has a degree of either 2 or 0. Hence it is a **full binary tree**.

For a complete binary tree, elements are stored in a level by level manner and filled from the leftmost side of the last level. Hence this a **complete binary tree**. Below is the array representation of the tree:

**Example 3:**

In the given binary tree node B has degree 1 which violates the property of full binary tree hence it is **not a full Binary tree**

For a complete binary tree, elements are stored in level by level manner and filled from the leftmost side of the last level. Hence this is a **complete binary tree**. Array representation of the binary tree is:

**Example 4:**

In the given binary tree node C has degree 1 which violates the property of a full binary tree hence it is **not a full Binary tree**

For a complete binary tree, elements are stored in level by level manner and filled from the leftmost side of the last level. Here node E violates the condition. Hence this is **not a complete binary tree**.

__Creation of Complete Binary Tree:__

We know a **complete binary tree** is a tree in which except for the last level (say **l**(all the other level has)**2l**) nodes and the nodes are lined up from left to right side.

It can be represented using an array. If the parent is it index **i** so the left child is at **2i+1** and the right child is at **2i+2**.

**Algorithm:**

For the creation of a **Complete Binary Tree**we require a queue data structure to keep track of the inserted nodes.

**Step 1:** Initialize the root with a new node when the tree is empty.

**Step 2:** If the tree is not empty then get the front element

- If the front element does not have a left child then set the left child to a new node
- If the right child is not present set the right child as a new node

**Step 3:** If the node has both the children then **pop** it from the queue.

**Step 4:** Enqueue the new data.

**Illustration:**

Consider the below array:

1. The 1st element will the root (value at index =

0)2. The next element (at index =

1) will be left and third element (index =2) will be right child of root3. fourth (index =

3) and fifth element (index =4) will be the left and right child of B node4. Next element (index =

5) will be left child of the node DThis is how complete binary tree is created.

**Implementation:** For the implementation of building a Complete Binary Tree from level order traversal is given in **this post**.

__Application of the Complete binary tree:__

- Heap Sort
- Heap sort based on data structure

** Check if a given binary tree is complete or not:** Follow

**this post**to check if the given binary tree is complete or not.