Insert at Middle Specific Position in DSA C++ - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after inserting at position 3 in a singly linked list

What is the printed state of the linked list after inserting the value 99 at position 3?

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

void insertAtPosition(Node*& head, int val, int pos) {
    Node* newNode = new Node(val);
    if (pos == 1) {
        newNode->next = head;
        head = newNode;
        return;
    }
    Node* temp = head;
    for (int i = 1; i < pos - 1 && temp != nullptr; i++) {
        temp = temp->next;
    }
    if (temp == nullptr) return; // position out of bounds
    newNode->next = temp->next;
    temp->next = newNode;
}

void printList(Node* head) {
    while (head) {
        std::cout << head->data << " -> ";
        head = head->next;
    }
    std::cout << "null" << std::endl;
}

int main() {
    Node* head = new Node(10);
    head->next = new Node(20);
    head->next->next = new Node(30);
    head->next->next->next = new Node(40);

    insertAtPosition(head, 99, 3);
    printList(head);
    return 0;
}

A 10 -> 99 -> 20 -> 30 -> 40 -> null

B 10 -> 20 -> 30 -> 99 -> 40 -> null

C 99 -> 10 -> 20 -> 30 -> 40 -> null

D 10 -> 20 -> 99 -> 30 -> 40 -> null

💡 Hint

Remember that position 3 means the new node should be the third node in the list.

The function inserts the new node at position 3, so the list becomes 10 -> 20 -> 99 -> 30 -> 40 -> null.

❓ Predict Output

intermediate

Result of inserting at position 1 (head) in a singly linked list

What is the printed state of the linked list after inserting the value 5 at position 1?

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

void insertAtPosition(Node*& head, int val, int pos) {
    Node* newNode = new Node(val);
    if (pos == 1) {
        newNode->next = head;
        head = newNode;
        return;
    }
    Node* temp = head;
    for (int i = 1; i < pos - 1 && temp != nullptr; i++) {
        temp = temp->next;
    }
    if (temp == nullptr) return; // position out of bounds
    newNode->next = temp->next;
    temp->next = newNode;
}

void printList(Node* head) {
    while (head) {
        std::cout << head->data << " -> ";
        head = head->next;
    }
    std::cout << "null" << std::endl;
}

int main() {
    Node* head = new Node(10);
    head->next = new Node(20);
    head->next->next = new Node(30);

    insertAtPosition(head, 5, 1);
    printList(head);
    return 0;
}

A 5 -> 20 -> 10 -> 30 -> null

B 10 -> 20 -> 30 -> 5 -> null

C 5 -> 10 -> 20 -> 30 -> null

D 10 -> 5 -> 20 -> 30 -> null

💡 Hint

Inserting at position 1 means the new node becomes the head.

The new node with value 5 is inserted at the head, so it appears first.

🔧 Debug

advanced

Identify the error in insertion at a specific position in singly linked list

What error will occur when running this code to insert at position 5 in a list of 3 nodes?

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

void insertAtPosition(Node*& head, int val, int pos) {
    Node* newNode = new Node(val);
    if (pos == 1) {
        newNode->next = head;
        head = newNode;
        return;
    }
    Node* temp = head;
    for (int i = 1; i < pos - 1; i++) {
        temp = temp->next;
    }
    newNode->next = temp->next;
    temp->next = newNode;
}

int main() {
    Node* head = new Node(1);
    head->next = new Node(2);
    head->next->next = new Node(3);

    insertAtPosition(head, 99, 5);
    return 0;
}

A Runtime error: segmentation fault due to dereferencing nullptr

B Compilation error: missing semicolon

C Logical error: node inserted at wrong position

D No error, insertion succeeds

💡 Hint

Check what happens when temp becomes nullptr in the loop.

The loop does not check if temp is nullptr before accessing temp->next, causing a crash.

❓ Predict Output

advanced

Output after inserting at position beyond list length

What is the printed state of the linked list after attempting to insert 50 at position 10 in a list of 4 nodes?

DSA C++

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

void insertAtPosition(Node*& head, int val, int pos) {
    Node* newNode = new Node(val);
    if (pos == 1) {
        newNode->next = head;
        head = newNode;
        return;
    }
    Node* temp = head;
    for (int i = 1; i < pos - 1 && temp != nullptr; i++) {
        temp = temp->next;
    }
    if (temp == nullptr) return; // position out of bounds
    newNode->next = temp->next;
    temp->next = newNode;
}

void printList(Node* head) {
    while (head) {
        std::cout << head->data << " -> ";
        head = head->next;
    }
    std::cout << "null" << std::endl;
}

int main() {
    Node* head = new Node(1);
    head->next = new Node(2);
    head->next->next = new Node(3);
    head->next->next->next = new Node(4);

    insertAtPosition(head, 50, 10);
    printList(head);
    return 0;
}

A 1 -> 2 -> 3 -> 4 -> 50 -> null

B 1 -> 2 -> 3 -> 4 -> null

C 50 -> 1 -> 2 -> 3 -> 4 -> null

D Runtime error: segmentation fault

💡 Hint

Insertion should not happen if position is beyond list length.

The function returns early if position is invalid, so list remains unchanged.

🧠 Conceptual

expert

Time complexity of inserting at a specific position in singly linked list

What is the time complexity of inserting a node at position k in a singly linked list of length n?

A O(k) because we traverse to the (k-1)th node before insertion

B O(1) because insertion is always at the head or tail

C O(n^2) because each insertion requires traversing the entire list multiple times

D O(log n) because we can use binary search to find the position

💡 Hint

Think about how many nodes you must visit to reach position k.

To insert at position k, you must traverse k-1 nodes, so time is proportional to k.

Practice

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

easy

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

B. Adding a new node exactly at the given position counting from the head

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

D. Replacing the node at the given position with a new node

Solution

1. Step 1: Understand insertion position

   Inserting at a specific middle position means placing the new node exactly at that position counting from the start (head) of the list.

2. Step 2: Differentiate from other insertions

   Insertion at start or end is different; here we insert in between nodes without replacing any node.

3. Final Answer:

   Adding a new node exactly at the given position counting from the head -> Option B

4. Quick Check:

   Insert at position means place new node there [OK]

Hint: Insert means add new node at exact position, not replace [OK]

Common Mistakes:

• Confusing insertion with replacement
• Thinking insertion only happens at start or end
• Ignoring position counting from head

2. Which of the following C++ code snippets correctly inserts a new node with value 10 at position 3 in a singly linked list?

easy

A. Node* temp = head; for(int i=1; i<2; i++) temp = temp->next; Node* newNode = new Node(10); newNode->next = temp->next; temp->next = newNode;

B. Node* newNode = new Node(10); newNode->next = head; head = newNode;

C. Node* temp = head; for(int i=1; i<3; i++) temp = temp->next; Node* newNode = new Node(10); newNode->next = temp->next; temp->next = newNode;

D. Node* newNode = new Node(10); head->next = newNode; newNode->next = NULL;

Solution

1. Step 1: Identify correct loop for position 3 insertion

   To insert at position 3, we stop at node 2 (position 2), so loop runs i=1 to i<2.

2. Step 2: Check pointer updates

   New node's next points to temp->next, then temp->next updated to new node. This correctly inserts at position 3.

3. Final Answer:

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

4. Quick Check:

   Loop stops at position before insertion [OK]

Hint: Loop till position-1, then insert new node [OK]

Common Mistakes:

• Looping one extra time causing wrong position
• Not updating newNode->next before temp->next
• Inserting at wrong position due to loop error

3. What will be the linked list after inserting value 15 at position 2 in the list: 5 -> 10 -> 20 -> null?

Node* head = new Node(5);
head->next = new Node(10);
head->next->next = new Node(20);

// Insert 15 at position 2
Node* temp = head;
for(int i=1; i<1; i++) temp = temp->next;
Node* newNode = new Node(15);
newNode->next = temp->next;
temp->next = newNode;

medium

A. 5 -> 15 -> 10 -> 20 -> null

B. 15 -> 5 -> 10 -> 20 -> null

C. 5 -> 10 -> 15 -> 20 -> null

D. 5 -> 10 -> 20 -> 15 -> null

Solution

1. Step 1: Trace loop to find insertion point

   Loop runs i=1 to i<1, so no iteration. temp points to head (value 5).

2. Step 2: Insert new node after temp

   New node 15 points to temp->next (10), then temp->next points to new node. List becomes 5 -> 15 -> 10 -> 20 -> null.

3. Final Answer:

   5 -> 15 -> 10 -> 20 -> null -> Option A

4. Quick Check:

   Insert after node 1 gives new node at position 2 [OK]

Hint: Loop zero times means insert after head [OK]

Common Mistakes:

• Miscounting loop iterations
• Inserting at wrong position
• Not updating pointers correctly

4. Identify the error in the following code that tries to insert a node with value 25 at position 4 in a linked list:

Node* temp = head;
for(int i=1; i<=4; i++) {
  temp = temp->next;
}
Node* newNode = new Node(25);
newNode->next = temp->next;
temp->next = newNode;

medium

A. No error, code works fine

B. New node's next pointer is not set correctly

C. New node is not created properly

D. Loop runs too many times causing temp to become NULL

Solution

1. Step 1: Analyze loop iterations

   Loop runs i=1 to i<=4 (4 times), moving temp 4 nodes ahead. For insertion at position 4, temp should stop at position 3.

2. Step 2: Consequence of loop error

   Temp moves too far, possibly NULL, causing runtime error when accessing temp->next.

3. Final Answer:

   Loop runs too many times causing temp to become NULL -> Option D

4. Quick Check:

   Loop must run position-2 times, not position times [OK]

Hint: Loop till position-2 to avoid NULL pointer [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Not checking for NULL before accessing next
• Incorrect loop range for insertion

5. Given a singly linked list with nodes: 1 -> 3 -> 5 -> 7 -> null, write the steps to insert a new node with value 4 at position 3. What will be the final list?

hard

A. 1 -> 4 -> 3 -> 5 -> 7 -> null

B. 4 -> 1 -> 3 -> 5 -> 7 -> null

C. 1 -> 3 -> 4 -> 5 -> 7 -> null

D. 1 -> 3 -> 5 -> 7 -> 4 -> null

Solution

1. Step 1: Traverse to node before position 3

   Start at head (1), move to next node (3) which is position 2. Stop here to insert after this node.

2. Step 2: Insert new node with value 4

   Create new node 4, set its next to current node's next (5), then update current node's next to new node.

3. Final Answer:

   1 -> 3 -> 4 -> 5 -> 7 -> null -> Option C

4. Quick Check:

   Insert at position 3 places new node between 3 and 5 [OK]

Hint: Stop at position-1 node, insert new node after it [OK]

Common Mistakes:

• Inserting at wrong position
• Not updating pointers correctly
• Confusing position counting from 0 or 1