Clone Linked List with Random Pointer in DSA C

Mental Model

We create a copy of each node and link them so that the new nodes are placed right after the original ones. Then we fix the random pointers of the new nodes by using the original nodes' random pointers.

Analogy: Imagine you have a row of houses (nodes) and each house has a secret friend (random pointer). You build a new house right next to each original house, then you ask each original house to tell its new neighbor who their secret friend is, so the new houses can connect to the right new friends.

Original list:
1 -> 2 -> 3 -> null
Random pointers:
1.random -> 3
2.random -> 1
3.random -> 2

After inserting clones:
1 -> 1' -> 2 -> 2' -> 3 -> 3' -> null
Random pointers to fix for clones.

Dry Run Walkthrough

Input: list: 1 -> 2 -> 3 with random pointers 1.random -> 3, 2.random -> 1, 3.random -> 2

Goal: Create a deep copy of the list where each new node has the same value and random pointer as the original, but all nodes are new objects.

Step 1: Insert clone nodes after each original node

1 -> 1' -> 2 -> 2' -> 3 -> 3' -> null

Why: We place clones next to originals to easily assign random pointers later.

Step 2: Assign random pointers for clone nodes using original nodes' random pointers

1.random -> 3, 1'.random -> 3'
2.random -> 1, 2'.random -> 1'
3.random -> 2, 3'.random -> 2'

Why: Clone's random pointer is set to original's random's next node (the clone).

Step 3: Separate the combined list into original and cloned lists

Original: 1 -> 2 -> 3 -> null
Clone: 1' -> 2' -> 3' -> null

Why: We restore the original list and extract the cloned list.

Result:

Cloned list: 1' -> 2' -> 3' -> null with random pointers 1'.random -> 3', 2'.random -> 1', 3'.random -> 2'

Annotated Code

DSA C

#include <stdio.h>
#include <stdlib.h>

// Node structure with value, next and random pointers
typedef struct Node {
    int val;
    struct Node* next;
    struct Node* random;
} Node;

// Create new node
Node* newNode(int val) {
    Node* node = (Node*)malloc(sizeof(Node));
    node->val = val;
    node->next = NULL;
    node->random = NULL;
    return node;
}

// Function to print list with random pointers
void printList(Node* head) {
    Node* curr = head;
    while (curr) {
        printf("%d", curr->val);
        if (curr->random) {
            printf("(rand:%d)", curr->random->val);
        } else {
            printf("(rand:null)");
        }
        if (curr->next) printf(" -> ");
        curr = curr->next;
    }
    printf(" -> null\n");
}

// Clone linked list with random pointer
Node* cloneList(Node* head) {
    if (!head) return NULL;

    Node* curr = head;
    // Step 1: Insert clone nodes after original nodes
    while (curr) {
        Node* copy = newNode(curr->val);
        copy->next = curr->next;
        curr->next = copy;
        curr = copy->next;
    }

    // Step 2: Assign random pointers for clone nodes
    curr = head;
    while (curr) {
        if (curr->random) {
            curr->next->random = curr->random->next;
        }
        curr = curr->next->next;
    }

    // Step 3: Separate original and clone lists
    curr = head;
    Node* cloneHead = head->next;
    Node* cloneCurr = cloneHead;
    while (curr) {
        curr->next = curr->next->next;
        if (cloneCurr->next) {
            cloneCurr->next = cloneCurr->next->next;
        } else {
            cloneCurr->next = NULL;
        }
        curr = curr->next;
        cloneCurr = cloneCurr->next;
    }

    return cloneHead;
}

int main() {
    // Create original list 1->2->3
    Node* n1 = newNode(1);
    Node* n2 = newNode(2);
    Node* n3 = newNode(3);
    n1->next = n2;
    n2->next = n3;

    // Setup random pointers
    n1->random = n3; // 1.random -> 3
    n2->random = n1; // 2.random -> 1
    n3->random = n2; // 3.random -> 2

    printf("Original list:\n");
    printList(n1);

    Node* cloned = cloneList(n1);

    printf("Cloned list:\n");
    printList(cloned);

    return 0;
}

while (curr) { Node* copy = newNode(curr->val); copy->next = curr->next; curr->next = copy; curr = copy->next; }

Insert clone nodes right after each original node to interleave lists

while (curr) { if (curr->random) { curr->next->random = curr->random->next; } curr = curr->next->next; }

Assign clone's random pointer to original's random's clone

while (curr) { curr->next = curr->next->next; if (cloneCurr->next) { cloneCurr->next = cloneCurr->next->next; } else { cloneCurr->next = NULL; } curr = curr->next; cloneCurr = cloneCurr->next; }

Restore original list and extract cloned list by fixing next pointers

Output

Original list:

1(rand:3) -> 2(rand:1) -> 3(rand:2) -> null

Cloned list:

1(rand:3) -> 2(rand:1) -> 3(rand:2) -> null

Complexity Analysis

Time: O(n) because we traverse the list a constant number of times proportional to the number of nodes

Space: O(1) extra space because we do not use additional data structures proportional to input size, only pointers manipulation

vs Alternative: Compared to using a hash map to store original-to-clone mapping which uses O(n) extra space, this method is more space efficient

Edge Cases

Empty list (head is NULL)

Function returns NULL immediately without errors

DSA C

if (!head) return NULL;

List with one node whose random pointer is NULL

Cloned list has one node with random pointer NULL

DSA C

if (curr->random) { curr->next->random = curr->random->next; }

Random pointers pointing to self

Cloned node's random pointer points to itself clone

DSA C

curr->next->random = curr->random->next;

When to Use This Pattern

When you see a linked list with an extra random pointer and need to clone it deeply, use the interleaving clone nodes pattern to assign random pointers without extra space.

Common Mistakes

Mistake: Not inserting clone nodes between original nodes, making it hard to assign random pointers correctly
Fix: Interleave clone nodes immediately after original nodes before assigning random pointers

Mistake: Assigning clone random pointers directly from original random pointers without mapping to clones
Fix: Set clone random pointer to original random's next node (the clone)

Mistake: Not restoring the original list after cloning, leaving the list corrupted
Fix: Separate the combined list back into original and cloned lists by fixing next pointers

Summary

Creates a deep copy of a linked list where each node has a random pointer.

Use when you need to clone complex linked lists with random pointers efficiently.

Interleave cloned nodes with original nodes to assign random pointers without extra space.

Practice

1. What is the main challenge when cloning a linked list with a random pointer?

easy

A. Maintaining the correct random pointer connections in the cloned list

B. Copying the data values of each node

C. Reversing the linked list before cloning

D. Sorting the linked list nodes

Solution

1. Step 1: Understand the linked list structure

   A linked list with random pointers has two pointers per node: next and random. The random pointer can point to any node or null.

2. Step 2: Identify cloning challenge

   While copying nodes is straightforward, correctly assigning random pointers in the cloned list is tricky because random pointers may point to nodes not yet cloned.

3. Final Answer:

   Maintaining the correct random pointer connections in the cloned list -> Option A

4. Quick Check:

   Random pointer cloning = main challenge [OK]

Hint: Focus on copying random pointers correctly [OK]

Common Mistakes:

• Ignoring random pointers during cloning
• Assuming random pointers are always null
• Copying only next pointers

2. Which of the following code snippets correctly creates a new node for cloning a linked list with random pointers in C?

easy

A. Node* newNode = (Node*)malloc(sizeof(Node)); newNode->data = oldNode->data; newNode->next = NULL; newNode->random = NULL;

B. Node newNode = malloc(sizeof(Node)); newNode.data = oldNode.data; newNode.next = NULL; newNode.random = NULL;

C. Node* newNode = malloc(Node); newNode->data = oldNode->data; newNode->next = NULL; newNode->random = NULL;

D. Node* newNode = (Node*)malloc(sizeof(Node)); newNode->data = oldNode->data; newNode->next = oldNode->next; newNode->random = oldNode->random;

Solution

1. Step 1: Check correct malloc usage

   In C, malloc requires sizeof(Node) to allocate memory. Casting to (Node*) is good practice.

2. Step 2: Verify initialization

   New node's data is copied, next and random pointers are set to NULL initially to avoid dangling pointers.

3. Final Answer:

   Node* newNode = (Node*)malloc(sizeof(Node)); newNode->data = oldNode->data; newNode->next = NULL; newNode->random = NULL; -> Option A

4. Quick Check:

   Correct malloc and initialization = Node* newNode = (Node*)malloc(sizeof(Node)); newNode->data = oldNode->data; newNode->next = NULL; newNode->random = NULL; [OK]

Hint: Use malloc with sizeof and initialize pointers to NULL [OK]

Common Mistakes:

• Missing sizeof in malloc
• Not casting malloc result
• Copying next and random pointers directly

3. Given the following linked list with random pointers:

Node1(data=1) -> Node2(data=2) -> NULL
Node1.random = Node2
Node2.random = Node2

What will be the random pointer of the cloned Node1 after cloning?

medium

A. Points to NULL

B. Points to cloned Node2

C. Points to original Node2

D. Points to cloned Node1

Solution

1. Step 1: Understand original random pointers

   Original Node1's random points to original Node2.

2. Step 2: Cloning preserves structure

   Cloned Node1's random should point to cloned Node2, not original nodes, to maintain independence.

3. Final Answer:

   Points to cloned Node2 -> Option B

4. Quick Check:

   Random pointer in clone = cloned target node [OK]

Hint: Random pointers in clone point to cloned nodes [OK]

Common Mistakes:

• Random pointer points to original node
• Random pointer set to NULL by mistake
• Confusing next and random pointers

4. Consider this buggy code snippet for cloning a linked list with random pointers:

while (curr != NULL) {
  Node* copy = (Node*)malloc(sizeof(Node));
  copy->data = curr->data;
  copy->next = curr->next;
  curr->next = copy;
  curr = curr->next;
}

What is the main error causing an infinite loop?

medium

A. Copy node's next pointer is not set to NULL

B. Updating curr to curr->next skips the newly inserted node

C. Not updating curr to curr->next->next after insertion

D. malloc size is incorrect

Solution

1. Step 1: Analyze pointer updates in loop

   After inserting copy node after curr, curr->next points to copy. Moving curr = curr->next moves to copy again.

2. Step 2: Identify infinite loop cause

   Loop never advances past copy node because curr is updated to copy, not original next node. It should be curr = curr->next->next.

3. Final Answer:

   Not updating curr to curr->next->next after insertion -> Option C

4. Quick Check:

   Advance curr by two nodes after insertion [OK]

Hint: Move curr two steps after inserting copy [OK]

Common Mistakes:

• Advancing curr by one node causes infinite loop
• Forgetting to link copy nodes properly
• Incorrect malloc size

5. You want to clone a linked list with random pointers in O(n) time and O(1) extra space. Which approach correctly achieves this?

hard

A. Reverse the original list, clone it, then reverse back

B. Use a hash map to store original to copy node mapping, then assign pointers

C. Copy nodes one by one and assign random pointers by searching original list each time

D. Create copy nodes interleaved with original nodes, set random pointers using original nodes, then separate lists

Solution

1. Step 1: Understand space and time constraints

   O(n) time and O(1) space means no extra data structures like hash maps.

2. Step 2: Use interleaving method

   Insert cloned nodes between original nodes, assign random pointers using original nodes, then detach cloned list. This avoids extra space.

3. Final Answer:

   Create copy nodes interleaved with original nodes, set random pointers using original nodes, then separate lists -> Option D

4. Quick Check:

   Interleaving nodes = O(1) space cloning [OK]

Hint: Interleave cloned nodes to avoid extra space [OK]

Common Mistakes:

• Using hash map increases space complexity
• Searching original list for random pointers is O(n^2)
• Reversing list does not help cloning random pointers