Insert at Middle Specific Position in DSA Rust - Execution Trace

Concept Flow - Insert at Middle Specific Position

Start

↓

Check if position is valid

Yes↓

Traverse list to position-1

↓

Create new node

↓

Adjust pointers to insert node

↓

Insertion complete

↓

End

No↓

Return error or no change

Check if position is valid, then move to the node before that position, create a new node, adjust pointers to insert it, else return error.

Execution Sample

DSA Rust

struct Node {
    data: i32,
    next: Option<Box<Node>>,
}

fn insert_at_position(head: &mut Option<Box<Node>>, data: i32, pos: usize) {
    // Insert node at pos (1-based)

Inserts a new node with given data at the specified 1-based position in the linked list.

Execution Table

Step	Current Position	Action	List State	Notes
1	Start (pos=3)	Check if pos=1	1 -> 2 -> 4 -> null	Position is 3, not 1, continue
2	pos=3	Traverse to node at pos-1=2	1 -> 2 -> 4 -> null	Current node at position 1: data=1
3	pos=3	Move to next node (pos=2)	1 -> 2 -> 4 -> null	Current node at position 2: data=2
4	pos=3	Create new node with data=3	1 -> 2 -> 4 -> null	New node created: 3 -> null
5	pos=3	Adjust pointers: new_node.next = current.next	1 -> 2 -> 4 -> null	new_node.next points to node with data=4
6	pos=3	current.next = new_node	1 -> 2 -> 3 -> 4 -> null	Node inserted at position 3
7	End	Return updated list	1 -> 2 -> 3 -> 4 -> null	Insertion complete
8	-	Exit	1 -> 2 -> 3 -> 4 -> null	pos was valid and insertion done

💡 Position 3 is valid and insertion completed successfully.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
pos	3	3	3	3	3	3	3
current_node	head (data=1)	data=1	data=2	data=2	data=2	data=2	data=2
new_node	None	None	None	data=3	data=3	data=3	data=3
list_state	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 4 -> null	1 -> 2 -> 3 -> 4 -> null	1 -> 2 -> 3 -> 4 -> null

Key Moments - 3 Insights

Why do we stop traversal at position-1 instead of position?

Because to insert a new node at position pos, we need to update the next pointer of the node at position pos-1 to point to the new node. This is shown in execution_table step 2 and 3 where we stop at node with data=2 before inserting.

What happens if the position is 1?

If position is 1, the new node becomes the new head. We set new_node.next to current head and update head to new_node. This is a special case not shown here but important to handle separately.

Why do we assign new_node.next = current.next before current.next = new_node?

To keep the rest of the list connected after insertion. If we didn't set new_node.next first, we would lose the link to the nodes after current. This is shown in step 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the data value of the new_node?

A 4

B 2

C 3

D None

💡 Hint

Check the 'Action' and 'List State' columns at step 4 in execution_table.

At which step does the list state change to include the new node?

A Step 3

B Step 6

C Step 4

D Step 2

💡 Hint

Look at the 'List State' column in execution_table to see when '3' appears.

If the position was 1, what would be the first action?

A Create new node and set its next to head

B Return error

C Traverse to position-1

D Do nothing

💡 Hint

Refer to key_moments about position 1 special case.

Concept Snapshot

Insert at Middle Specific Position in Linked List:
- Check if position is valid
- Traverse to node before position
- Create new node
- Set new_node.next = current.next
- Set current.next = new_node
- Special case: position 1 means new head
- Result: node inserted at given position

Full Transcript

This visual execution shows how to insert a new node at a specific middle position in a singly linked list. We start by checking if the position is valid. Then we traverse the list to the node just before the desired position. We create a new node with the given data. Next, we adjust pointers so the new node points to the next node, and the previous node points to the new node. This inserts the new node in the middle. The example inserts data=3 at position 3 in the list 1 -> 2 -> 4 -> null, resulting in 1 -> 2 -> 3 -> 4 -> null. Special cases like inserting at position 1 require updating the head pointer. The execution table and variable tracker show each step and how variables change. Key moments clarify common confusions about pointer adjustments and traversal stopping points. The quiz tests understanding of these steps.

Practice

1. What does inserting a node at a specific middle position in a linked list mean?

easy

A. Removing a node from the middle of the list

B. Adding a new node only at the start of the list

C. Adding a new node only at the end of the list

D. Adding a new node exactly between two existing nodes at a given position

Solution

1. Step 1: Understand linked list positions

   Positions in a linked list start from 1 at the head. Inserting at a middle position means placing a new node between existing nodes.

2. Step 2: Define insertion at middle

   Inserting at a specific middle position means the new node will be placed after the node at position (pos-1) and before the node at position pos.

3. Final Answer:

   Adding a new node exactly between two existing nodes at a given position -> Option D

4. Quick Check:

   Insert at middle = Add node between nodes [OK]

Hint: Middle insertion means placing node between two nodes [OK]

Common Mistakes:

• Confusing insertion with deletion
• Thinking insertion only happens at start or end
• Ignoring position indexing starting at 1

2. Which Rust code snippet correctly inserts a new node with value 10 at position 3 in a singly linked list?

easy

A. let mut current = &mut head;
for _ in 1..2 {
current = &mut current.as_mut().unwrap().next;
}
let new_node = Some(Box::new(Node { val: 10, next: current.take() }));
*current = new_node;

B. let mut current = &head;
for _ in 1..3 {
current = ¤t.unwrap().next;
}
current = Some(Box::new(Node { val: 10, next: None }));

C. head = Some(Box::new(Node { val: 10, next: head }));

D. let new_node = Node { val: 10, next: None };
head.next = Some(Box::new(new_node));

Solution

1. Step 1: Traverse to position before insertion

   The loop runs from 1 to 2 (pos-1) to reach node at position 2, using mutable references to modify links.

2. Step 2: Insert new node at position 3

   Create new node with value 10, set its next to current node's next, then update current's next to new node.

3. Final Answer:

   Code snippet in option A correctly inserts at position 3 -> Option A

4. Quick Check:

   Traverse to pos-1, link new node next to current next, update current next [OK]

Hint: Traverse to pos-1, then link new node [OK]

Common Mistakes:

• Using immutable references when mutable needed
• Incorrect loop range causing wrong position
• Not updating links properly causing list break

3. What is the output linked list after inserting value 99 at position 2 in the list 1 -> 2 -> 3 -> null using the following Rust code?

let mut head = Some(Box::new(Node { val: 1, next: Some(Box::new(Node { val: 2, next: Some(Box::new(Node { val: 3, next: None })) })) }));

let mut current = &mut head;
for _ in 1..1 {
    current = &mut current.as_mut().unwrap().next;
}
let new_node = Some(Box::new(Node { val: 99, next: current.as_mut().unwrap().next.take() }));
current.as_mut().unwrap().next = new_node;

// Print list code omitted for brevity

medium

A. 1 -> 99 -> 2 -> 3 -> null

B. 1 -> 2 -> 99 -> 3 -> null

C. 99 -> 1 -> 2 -> 3 -> null

D. 1 -> 2 -> 3 -> 99 -> null

Solution

1. Step 1: Traverse to node at position 1

   The loop runs from 1 to 1, which is empty, so no iteration; current points to head node with value 1.

2. Step 2: Insert new node after current

   New node with value 99 is created with next pointing to current's next (node with value 2). Then current's next is updated to new node.

3. Final Answer:

   1 -> 99 -> 2 -> 3 -> null -> Option A

4. Quick Check:

   Insert after position 1 adds 99 between 1 and 2 [OK]

Hint: Insert after pos-1 node, new node points to old next [OK]

Common Mistakes:

• Miscounting loop iterations causing wrong position
• Not updating next pointers correctly
• Confusing node values and positions

4. Identify the error in this Rust code snippet that tries to insert a node at position 2 in a singly linked list:

let mut current = &mut head;
for _ in 0..2 {
    current = &mut current.as_mut().unwrap().next;
}
let new_node = Some(Box::new(Node { val: 5, next: current.take() }));
*current = new_node;

medium

A. The code uses immutable references instead of mutable

B. The loop runs too many times, moving current past the insertion point

C. The new node's next pointer is not set correctly

D. The head pointer is not updated after insertion

Solution

1. Step 1: Analyze loop iteration count

   The loop runs from 0 to 1 (0..2), so two iterations, moving current two nodes ahead, which is past position 2.

2. Step 2: Understand insertion position

   To insert at position 2, current should point to node at position 1 (pos-1). Loop should run once, not twice.

3. Final Answer:

   The loop runs too many times, moving current past the insertion point -> Option B

4. Quick Check:

   Loop count must be pos-1 to insert correctly [OK]

Hint: Loop must run pos-1 times to reach insertion point [OK]

Common Mistakes:

• Off-by-one error in loop range
• Not using mutable references for insertion
• Incorrectly setting new node's next pointer

5. Given a singly linked list and a position that may be beyond the current list length, how should you modify the insertion logic in Rust to insert a node with value 50 at position 5 in the list 10 -> 20 -> 30 -> null?

hard

A. Return an error and do not insert if position is beyond length

B. Insert at the head regardless of position

C. Traverse until the end if position exceeds length, then insert at the end

D. Insert at position 3 instead of 5 automatically

Solution

1. Step 1: Check position against list length

   Since position 5 is beyond current length 3, traverse until the last node to insert at the end.

2. Step 2: Insert new node at end

   Once at last node, set its next pointer to new node with value 50 and next as None.

3. Final Answer:

   Traverse until the end if position exceeds length, then insert at the end -> Option C

4. Quick Check:

   Insert at end if position too large [OK]

Hint: If position too big, insert at list end [OK]

Common Mistakes:

• Crashing or panicking when position is too large
• Ignoring position and inserting at wrong place
• Not handling None next pointers properly