Why Doubly Linked List Over Singly Linked List in DSA Go - Quick Recap

Recall & Review

beginner

What is a doubly linked list?

A doubly linked list is a type of linked list where each node has two pointers: one to the next node and one to the previous node. This allows traversal in both directions.

beginner

What is the main difference between singly and doubly linked lists?

Singly linked lists have nodes with only one pointer to the next node, allowing traversal in one direction. Doubly linked lists have two pointers per node, enabling traversal forwards and backwards.

intermediate

Why might you choose a doubly linked list over a singly linked list?

You choose a doubly linked list when you need to traverse the list in both directions or delete nodes efficiently without needing to traverse from the head to find the previous node.

intermediate

What is a downside of using a doubly linked list compared to a singly linked list?

Doubly linked lists use more memory because each node stores an extra pointer to the previous node. They also have slightly more complex insertion and deletion logic.

intermediate

How does a doubly linked list improve deletion of a node compared to a singly linked list?

In a doubly linked list, you can delete a node directly if you have a pointer to it because you can access its previous node easily. In a singly linked list, you must traverse from the head to find the previous node before deletion.

Which pointer(s) does a node in a doubly linked list have?

A No pointers

B Pointer to next and previous nodes

C Pointer to previous node only

D Pointer to next node only

A doubly linked list node has two pointers: one to the next node and one to the previous node.

What is a key advantage of a doubly linked list over a singly linked list?

A Faster insertion at head only

B Uses less memory

C Simpler node structure

D Can traverse backwards easily

Doubly linked lists allow easy backward traversal because of the previous pointer.

Why might deletion be faster in a doubly linked list?

A Because you can access the previous node directly

B Because nodes have no pointers

C Because you only need to traverse forward

D Because nodes are stored in arrays

Direct access to the previous node allows deletion without traversal from the head.

What is a disadvantage of doubly linked lists?

A More memory usage per node

B Cannot traverse backwards

C No pointers to next node

D Cannot insert nodes

Each node stores an extra pointer, increasing memory usage.

In which scenario is a singly linked list preferred over a doubly linked list?

A When deletion speed is critical

B When backward traversal is required

C When memory is limited and only forward traversal is needed

D When nodes need two pointers

Singly linked lists use less memory and are simpler when only forward traversal is needed.

Explain why a doubly linked list might be chosen over a singly linked list in a real-world application.

Think about when you need to move both ways in a list and delete nodes quickly.

Describe the main differences in node structure and operations between singly and doubly linked lists.

Focus on pointers and how they affect list operations.

Practice

1. Why might you choose a doubly linked list over a singly linked list in a program?

easy

A. Because it uses less memory per node

B. Because it only stores one pointer per node

C. Because it is simpler to implement

D. Because it allows traversal in both directions easily

Solution

1. Step 1: Understand traversal capabilities

   A doubly linked list stores two pointers per node: one to the next node and one to the previous node, allowing movement forward and backward.

2. Step 2: Compare with singly linked list

   A singly linked list stores only one pointer to the next node, so it can only be traversed forward easily.

3. Final Answer:

   Because it allows traversal in both directions easily -> Option D

4. Quick Check:

   Traversal both ways = B [OK]

Hint: Doubly means two directions for traversal [OK]

Common Mistakes:

• Thinking doubly linked lists use less memory
• Assuming singly linked lists allow backward traversal
• Confusing simplicity of implementation

2. Which of the following is the correct way to declare a node struct for a doubly linked list in Go?

easy

A. type Node struct { data int; prev *Node; next *Node }

B. type Node struct { data int; next *Node }

C. type Node struct { data int; prev int; next int }

D. type Node struct { data int; next int }

Solution

1. Step 1: Identify required pointers for doubly linked list node

   A doubly linked list node must have pointers to both previous and next nodes, so it needs two pointers of type *Node.

2. Step 2: Check each option's struct fields

   type Node struct { data int; prev *Node; next *Node } correctly declares both prev *Node and next *Node. Others miss one or use wrong types.

3. Final Answer:

   type Node struct { data int; prev *Node; next *Node } -> Option A

4. Quick Check:

   Two pointers prev and next = A [OK]

Hint: Doubly linked node needs prev and next pointers [OK]

Common Mistakes:

• Using int instead of pointer for prev/next
• Missing prev pointer in struct
• Confusing singly linked list node with doubly linked

3. What will be the output of the following Go code snippet that uses a doubly linked list?

type Node struct {
    data int
    prev *Node
    next *Node
}

func printForward(head *Node) {
    for current := head; current != nil; current = current.next {
        fmt.Print(current.data, " ")
    }
}

func printBackward(tail *Node) {
    for current := tail; current != nil; current = current.prev {
        fmt.Print(current.data, " ")
    }
}

func main() {
    n1 := &Node{data: 1}
    n2 := &Node{data: 2}
    n3 := &Node{data: 3}

    n1.next = n2
    n2.prev = n1
    n2.next = n3
    n3.prev = n2

    printForward(n1)
    fmt.Println()
    printBackward(n3)
}

medium

A. 1 2 3 \n 1 2 3

B. 3 2 1 \n 1 2 3

C. 1 2 3 \n 3 2 1

D. 3 2 1 \n 3 2 1

Solution

1. Step 1: Trace printForward function

   Starting at n1 (data=1), it prints 1, then moves to n2 (2), then n3 (3), then nil. Output: "1 2 3 "

2. Step 2: Trace printBackward function

   Starting at n3 (3), it prints 3, then moves to n2 (2), then n1 (1), then nil. Output: "3 2 1 "

3. Final Answer:

   1 2 3 \n 3 2 1 -> Option C

4. Quick Check:

   Forward then backward print = A [OK]

Hint: Forward prints from head, backward from tail [OK]

Common Mistakes:

• Confusing prev and next pointers in loops
• Printing backward list in forward order
• Missing nil check causing runtime error

4. The following Go code tries to insert a new node after a given node in a doubly linked list. What is the bug?

func insertAfter(node *Node, newNode *Node) {
    newNode.next = node.next
    node.next = newNode
    newNode.prev = node
}

medium

A. It does not set node.next correctly

B. It does not update the prev pointer of the node after newNode

C. It does not set newNode.prev correctly

D. It does not set newNode.next correctly

Solution

1. Step 1: Analyze pointer updates in insertAfter

   The code sets newNode.next to node.next and newNode.prev to node, and updates node.next to newNode.

2. Step 2: Identify missing pointer update

   If node.next was not nil, the previous pointer of node.next (the node after newNode) should be updated to newNode. This step is missing.

3. Final Answer:

   It does not update the prev pointer of the node after newNode -> Option B

4. Quick Check:

   Missing prev update of next node = C [OK]

Hint: Always update prev pointer of next node after insertion [OK]

Common Mistakes:

• Forgetting to update next node's prev pointer
• Confusing prev and next pointers
• Assuming node.next is always nil

5. You want to implement a feature that requires frequent backward traversal and deletion of nodes from the middle of a list. Why is a doubly linked list better than a singly linked list for this task?

hard

A. Because doubly linked lists allow backward traversal and easy access to previous nodes for deletion

B. Because singly linked lists use less memory and are faster for deletion

C. Because doubly linked lists do not need pointers to previous nodes

D. Because singly linked lists allow backward traversal without extra pointers

Solution

1. Step 1: Understand backward traversal requirement

   Backward traversal means moving from a node to its previous node, which singly linked lists cannot do easily because they only have next pointers.

2. Step 2: Understand deletion from middle nodes

   To delete a node, you need to update the previous node's next pointer. Doubly linked lists have direct access to previous nodes, making deletion easier and efficient.

3. Final Answer:

   Because doubly linked lists allow backward traversal and easy access to previous nodes for deletion -> Option A

4. Quick Check:

   Backward traversal and deletion ease = D [OK]

Hint: Backward traversal needs prev pointers for easy deletion [OK]

Common Mistakes:

• Thinking singly linked lists can traverse backward easily
• Ignoring need to update previous node on deletion
• Assuming doubly linked lists use less memory