2. Tree Concept

2.1 Basic Theory

2.1.1 Definition of a Tree

A Tree is a non-linear hierarchical data structure composed of nodes and edges.

• Every Tree has a root (root node).
• The root can have children.
• A node with no children is called a leaf.
• Each node and its descendants can form a subtree.

2.1.2 Important Terms in a Tree

• Root → the topmost node.
• Parent → a node that has children.
• Child → a node derived from a parent.
• Leaf → a node without children.
• Subtree → a small tree formed from a node and its descendants.

2.1.3 Binary Tree

A Binary Tree is a tree data structure in which each node has at most two children (left and right).

• There are no special rules for placing node values.
• Used for hierarchical representation, data structures like heaps, expression trees, and implementing search or traversal algorithms.
• Nodes in a Binary Tree can have 0, 1, or 2 children.

Characteristics of a Binary Tree:

• Height of the tree: the number of levels from the root to the farthest leaf.
• Leaf node: a node that has no children.

2.2 Introduction to Stack and Queue

2.2.1 Stack

A LIFO (Last In First Out) data structure → the last data in is the first one out. Example: a stack of plates in a cafeteria. The last plate placed on top will be the first one taken.

Main operations:

• push(x) → adds data to the top of the stack.
• pop() → removes data from the top of the stack.

Application in Trees Used in DFS (Depth First Search) or Preorder, Inorder, Postorder traversals. When we enter a child node, the current node is stored on the stack. After finishing with the child, we pop from the stack to continue.

2.2.2 Queue

A FIFO (First In First Out) data structure → the first data in is the first one out. Example: a minimarket cashier line, the person who arrives first will be served first.

Main operations:

• enqueue(x) → inserts data at the back of the queue.
• dequeue() → removes data from the front of the queue.

Application in Trees Used in BFS (Breadth First Search) or Level Order traversal. When visiting a node, its children are added to the queue, then processed one by one in order.

2.3 Traversal in Trees

Traversal is the process of visiting each node in a tree.

2.3.1 Inorder (Left, Root, Right)

• visit the left, then the middle (root), then the right. The result is usually data sorted in ascending order.

// Inorder (Left - Root - Right)
void inorder(Node* root) {
   if (root == nullptr) return;
   inorder(root->left);
   cout << root->data << " ";
   inorder(root->right);
}

2.3.2 Preorder (Root, Left, Right)

• visit the middle first, then the left, then the right. Used to represent the structure of the tree.

// Preorder (Root - Left - Right)
void preorder(Node* root) {
   if (root == nullptr) return;
   cout << root->data << " ";
   preorder(root->left);
   preorder(root->right);
}

2.3.3 Postorder (Left, Right, Root)

• visit the left first, then the right, finally the middle. Suitable if you want to delete all contents of the tree.

// Postorder (Left - Right - Root)
void postorder(Node* root) {
   if (root == nullptr) return;
   postorder(root->left);
   postorder(root->right);
   cout << root->data << " ";
}

2.3.4 Level Order

• Visit nodes level by level from top to bottom, left to right.
• Usually uses a queue.

void levelOrder(Node* root) {
   if (root == nullptr) return;
   queue<Node*> q;
   q.push(root);
   while (!q.empty()) {
   Node* temp = q.front();
   q.pop();
   cout << temp->data << " ";
   if (temp->left) q.push(temp->left);
   if (temp->right) q.push(temp->right);
   }
}

2.3.5 Traversal Illustration Example

     50
    /  \
   /    \
  30     70
 / \    / \
20 40  60 80

• Inorder: 20, 30, 40, 50, 60, 70, 80
• Preorder: 50, 30, 20, 40, 70, 60, 80
• Postorder: 20, 40, 30, 60, 80, 70, 50
• Level Order: 50, 30, 70, 20, 40, 60, 80

References

• GeeksforGeeks, “Binary Search Tree (BST),” [Online]. Available: https://www.geeksforgeeks.org/binary-search-tree-data-structure/. [Accessed: 20-August-2025]
• Programiz, “Tree Data Structure in C++,” [Online]. Available: https://www.programiz.com/dsa/binary-search-tree. [Accessed: 20-August-2025]