Inorder, Preorder, and Postorder Traversal
💡 Concept Name
Tree Traversals – Different ways to visit each node in a tree, mainly using depth-first orders: inorder, preorder, and postorder.
📘 Quick Intro
Traversing a binary tree means visiting all nodes in a certain sequence. The three common DFS traversal orders are:
Inorder: Left subtree → Root → Right subtree
Preorder: Root → Left subtree → Right subtree
Postorder: Left subtree → Right subtree → Root
🧠 Analogy / Short Story
Think of cleaning a house: Preorder is greeting the owner first before checking rooms; Inorder is visiting all rooms then talking to the owner; Postorder is cleaning rooms first, and only then saying goodbye to the owner.
🔧 Technical Explanation
- 📌 Inorder: Visit left child, then root, then right child.
- 📌 Preorder: Visit root first, then left and right children.
- 📌 Postorder: Visit left and right children before the root.
- 📊 Inorder traversal of a Binary Search Tree (BST) outputs nodes in sorted order.
- 🔁 These are recursive depth-first traversal methods.
🎯 Purpose & Use Case
- ✅ Inorder traversal is useful for retrieving sorted data from BSTs.
- ✅ Preorder is often used for copying or serializing trees.
- ✅ Postorder traversal is handy when deleting or freeing nodes.
- ✅ All are essential for recursive algorithms and expression tree evaluations.
💻 Real Code Example
public class TreeNode
{
public int Value;
public TreeNode? Left, Right;
public TreeNode(int value) { Value = value; }
public void Inorder()
{
Left?.Inorder();
Console.Write(Value + " ");
Right?.Inorder();
}
public void Preorder()
{
Console.Write(Value + " ");
Left?.Preorder();
Right?.Preorder();
}
public void Postorder()
{
Left?.Postorder();
Right?.Postorder();
Console.Write(Value + " ");
}
}
❓ Interview Q&A
Q1: What is inorder traversal?
A: Visiting the left subtree, then the node, and finally the right subtree.
Q2: What is preorder traversal?
A: Visiting the node first, then the left subtree, followed by the right subtree.
Q3: What is postorder traversal?
A: Visiting the left subtree, right subtree, and then the node.
Q4: Which traversal is used to get nodes in sorted order in a BST?
A: Inorder traversal.
Q5: When is preorder traversal useful?
A: For copying trees or prefix expression evaluations.
Q6: What is a common use of postorder traversal?
A: For deleting trees or postfix expression evaluations.
Q7: Can these traversals be implemented recursively and iteratively?
A: Yes, both methods are possible.
Q8: What data structure is often used in iterative traversals?
A: A stack.
Q9: How does preorder traversal start?
A: It starts by visiting the root node.
Q10: Which traversal visits the root node last?
A: Postorder traversal.
📝 MCQs
Q1. What is the order of inorder traversal?
- Node, Left, Right
- Left, Node, Right
- Left, Right, Node
- Right, Left, Node
Q2. What is the order of preorder traversal?
- Left, Node, Right
- Node, Left, Right
- Left, Right, Node
- Right, Node, Left
Q3. What is the order of postorder traversal?
- Node, Left, Right
- Left, Node, Right
- Left, Right, Node
- Right, Left, Node
Q4. Which traversal produces sorted nodes in a BST?
- Preorder
- Inorder
- Postorder
- Level order
Q5. Which traversal visits the root node first?
- Preorder
- Inorder
- Postorder
- Level order
Q6. Which traversal visits the root node last?
- Preorder
- Inorder
- Postorder
- Level order
Q7. What data structure is used for iterative traversal?
- Queue
- Stack
- Heap
- Array
Q8. What is a use case for preorder traversal?
- Deleting a tree
- Copying a tree
- Sorting nodes
- Searching nodes
Q9. What is a use case for postorder traversal?
- Copying a tree
- Deleting a tree
- Sorting nodes
- Searching nodes
Q10. Can tree traversals be implemented recursively?
- No
- Yes
- Only inorder
- Only preorder
💡 Bonus Insight
Easy way to remember:
Inorder = Sorted order,
Preorder = Copy order,
Postorder = Cleanup order.
You can also implement all three traversals with minor changes in recursive calls or use stacks for iterative solutions.
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