Push Using Linked List Node in DSA Go - Time & Space Complexity

Time Complexity: Push Using Linked List Node

O(1)

Understanding Time Complexity

We want to understand how long it takes to add a new node at the start of a linked list.

How does the time needed change as the list grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

type Node struct {
    data int
    next *Node
}

func push(head **Node, newData int) {
    newNode := &Node{data: newData}
    newNode.next = *head
    *head = newNode
}

This code adds a new node at the front of a linked list by adjusting pointers.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating a new node and changing pointers.
• How many times: This happens once per push call, no loops or recursion inside.

How Execution Grows With Input

Adding a node always takes the same steps regardless of list size.

Input Size (n)	Approx. Operations
10	3 steps
100	3 steps
1000	3 steps

Pattern observation: The number of steps stays the same no matter how big the list is.

Final Time Complexity

Time Complexity: O(1)

This means adding a node at the front takes a fixed amount of time, no matter the list size.

Common Mistake

[X] Wrong: "Adding a node takes longer as the list grows because we have more nodes."

[OK] Correct: We only change the first node's pointer, so the rest of the list size does not affect the time.

Interview Connect

Knowing that pushing to a linked list is quick helps you explain efficient data structure operations clearly.

Self-Check

"What if we had to add a node at the end of the linked list instead? How would the time complexity change?"

Practice

1. What does the push operation do in a linked list when using a node?

easy

A. Deletes the entire linked list

B. Removes the last node from the list

C. Searches for a node with a specific value

D. Adds a new node at the beginning of the list

Solution

1. Step 1: Understand push operation in linked list

   The push operation adds a new node at the start (head) of the linked list.

2. Step 2: Effect of push on list structure

   The new node becomes the head, and its next pointer points to the previous head node.

3. Final Answer:

   Adds a new node at the beginning of the list -> Option D

4. Quick Check:

   Push = Add at head [OK]

Hint: Push always adds node at the start of the list [OK]

Common Mistakes:

• Thinking push adds at the end
• Confusing push with pop
• Assuming push deletes nodes

2. Which of the following is the correct syntax to create a new node and push it onto a linked list in Go?

easy

A. newNode := Node{data: 5}; head.next = newNode

B. newNode := &Node{data: 5, next: head}; head = newNode

C. newNode := &Node{5}; head = newNode.next

D. newNode := Node{data: 5, next: nil}; head = newNode.next

Solution

1. Step 1: Create a new node with data and next pointer

   In Go, use &Node{data: 5, next: head} to create a new node pointing to current head.

2. Step 2: Update head to new node

   Assign head = newNode to make newNode the new head of the list.

3. Final Answer:

   newNode := &Node{data: 5, next: head}; head = newNode -> Option B

4. Quick Check:

   New node points to head, then head updated [OK]

Hint: New node's next points to head, then head updated [OK]

Common Mistakes:

• Not using pointer (&) for new node
• Assigning head.next instead of head
• Setting next to nil instead of head

3. What will be the output linked list after executing this Go code?

type Node struct {
  data int
  next *Node
}

func main() {
  var head *Node = nil
  head = &Node{data: 1, next: head}
  head = &Node{data: 2, next: head}
  head = &Node{data: 3, next: head}
  printList(head)
}

func printList(node *Node) {
  for node != nil {
    fmt.Print(node.data, " -> ")
    node = node.next
  }
  fmt.Print("null")
}

medium

A. 3 -> 2 -> 1 -> null

B. 1 -> 2 -> 3 -> null

C. null

D. 1 -> null

Solution

1. Step 1: Trace node insertions

   First node with data 1 is pushed, head points to node 1.
   Then node 2 is pushed, head points to node 2, next points to node 1.
   Then node 3 is pushed, head points to node 3, next points to node 2.

2. Step 2: Print list from head

   Prints 3 -> 2 -> 1 -> null as nodes are added at front.

3. Final Answer:

   3 -> 2 -> 1 -> null -> Option A

4. Quick Check:

   Push adds at head, so list reversed [OK]

Hint: Push adds at head, so last pushed is first in list [OK]

Common Mistakes:

• Assuming push adds at tail
• Confusing order of nodes
• Ignoring nil termination

4. Identify the error in this Go code that tries to push a new node onto a linked list:

type Node struct {
  data int
  next *Node
}

func push(head *Node, data int) {
  newNode := &Node{data: data, next: head}
  head = newNode
}

func main() {
  var head *Node = nil
  push(head, 10)
  // printList(head) expected to show 10 -> null
}

medium

A. head is passed by value, so main's head is not updated

B. newNode is not created correctly with & operator

C. next pointer should be nil, not head

D. Function push should return void, not update head

Solution

1. Step 1: Analyze parameter passing

   head is passed by value to push, so changes inside push do not affect main's head.

2. Step 2: Effect on linked list head

   Assigning head = newNode inside push only changes local copy, main's head remains nil.

3. Final Answer:

   head is passed by value, so main's head is not updated -> Option A

4. Quick Check:

   Pointer must be passed by reference or returned [OK]

Hint: Pass head pointer by reference or return new head [OK]

Common Mistakes:

• Assuming pointer changes reflect outside function
• Not returning updated head
• Misunderstanding Go parameter passing

5. Given a linked list with nodes containing integers, how would you modify the push function to avoid memory leaks and ensure the list is correctly updated in Go?

hard

A. Use a slice instead of linked list for better memory management

B. Use a global variable for head instead of passing it

C. Make push return the new head node and assign it to head in main

D. Set newNode.next to nil before assigning head

Solution

1. Step 1: Understand pointer update requirement

   In Go, to update head outside push, function must return new head node.

2. Step 2: Assign returned head in main

   Call head = push(head, data) to update head correctly and avoid dangling pointers.

3. Final Answer:

   Make push return the new head node and assign it to head in main -> Option C

4. Quick Check:

   Return new head to update list safely [OK]

Hint: Return new head from push and assign it outside [OK]

Common Mistakes:

• Not returning updated head
• Using global variables unnecessarily
• Ignoring pointer semantics in Go