Intersection Point of Two Linked Lists in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the intersection point of two linked lists?

The intersection point is the node where two linked lists merge and share the same next nodes onwards.

beginner

Why can't we find the intersection point by comparing node values?

Because nodes may have the same values but be different nodes. Intersection means the exact same node in memory, not just equal values.

intermediate

What is the main idea behind the two-pointer approach to find the intersection point?

Use two pointers starting at each list head. When a pointer reaches the end, switch it to the other list's head. They meet at the intersection or end (null).

intermediate

What is the time complexity of the two-pointer method for finding the intersection point?

The time complexity is O(m + n), where m and n are the lengths of the two linked lists.

beginner

What happens if two linked lists do not intersect when using the two-pointer approach?

Both pointers will eventually become null after traversing both lists, and the function returns null indicating no intersection.

What does the intersection point of two linked lists represent?

A The node with the same value in both lists

B The last node of the shorter list

C The first node of the longer list

D The node where both lists share the same memory address

Intersection means the exact same node in memory, not just equal values.

In the two-pointer approach, what do you do when a pointer reaches the end of its list?

A Move it back to the start of the same list

B Stop and return null

C Switch it to the head of the other list

D Delete the pointer

Switching pointers to the other list head helps align traversal lengths.

What is the time complexity of finding the intersection point using the two-pointer method?

A O(m * n)

B O(m + n)

C O(1)

D O(max(m, n)^2)

The method traverses each list at most twice, so O(m + n).

If two linked lists do not intersect, what will the two-pointer method return?

A Null

B The last node of the shorter list

C The first node with the same value

D The head of the longer list

If no intersection, pointers become null after full traversal.

Why is comparing node values not enough to find the intersection point?

A Because nodes with same values can be different nodes

B Because values are always unique

C Because values can be different at the intersection

D Because values are not stored in nodes

Intersection means same node object, not just same value.

Explain how the two-pointer approach finds the intersection point of two linked lists.

Think about how switching pointers helps align traversal.

Describe what the intersection point means in the context of linked lists and why node identity matters.

Focus on node memory address, not values.

Practice

1. What does the term intersection point of two linked lists mean?

easy

A. The last node of each linked list.

B. The node where the values of both linked lists are equal.

C. The first node of the longer linked list.

D. The node where both linked lists share the same reference (memory address).

Solution

1. Step 1: Understand linked list nodes

   Each node in a linked list has a value and a reference (address) to the next node.

2. Step 2: Define intersection point

   The intersection point is where two linked lists share the exact same node reference, not just the same value.

3. Final Answer:

   The node where both linked lists share the same reference (memory address). -> Option D

4. Quick Check:

   Intersection means shared node reference = A [OK]

Hint: Intersection means same node object, not just same value [OK]

Common Mistakes:

• Confusing same value with same node reference
• Thinking intersection is the last node always
• Assuming intersection means equal length lists

2. Which of the following C++ code snippets correctly declares a singly linked list node structure for intersection detection?

easy

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

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

C. struct Node { int data; Node* next(){}; };

D. struct Node { int data; Node next(); };

Solution

1. Step 1: Understand pointer usage in C++ structs

   A linked list node must have a pointer to the next node, so Node* next; is correct.

2. Step 2: Analyze options

   struct Node { int data; Node* next; }; uses Node* next; which is correct. Options A, B, and D misuse pointer or function syntax.

3. Final Answer:

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

4. Quick Check:

   Pointer to next node = C [OK]

Hint: Use pointer (*) for next node in struct [OK]

Common Mistakes:

• Forgetting the * pointer for next node
• Using function syntax instead of pointer
• Declaring next as object instead of pointer

3. Given two linked lists intersecting at node with value 7, what will the following code print?

Node* getIntersectionNode(Node* headA, Node* headB) {
    Node* a = headA;
    Node* b = headB;
    while (a != b) {
        a = a ? a->next : headB;
        b = b ? b->next : headA;
    }
    return a;
}

// Lists:
// A: 3 -> 6 -> 9 -> 7 -> 10
// B: 1 -> 7 -> 10

Node* intersection = getIntersectionNode(headA, headB);
if (intersection) std::cout << intersection->data << std::endl;
else std::cout << -1 << std::endl;

medium

A. -1

B. 10

C. 7

D. 3

Solution

1. Step 1: Understand the algorithm

   This code uses two pointers traversing both lists; when one reaches end, it switches to the other list's head. They meet at intersection.

2. Step 2: Trace the pointers

   Both pointers meet at node with value 7, which is the intersection point.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Intersection node data = 7 [OK]

Hint: Two-pointer switch method finds intersection node [OK]

Common Mistakes:

• Assuming intersection is node with value 10
• Printing -1 when intersection exists
• Confusing node value with node reference

4. Identify the bug in this code snippet that tries to find the intersection point of two linked lists:

Node* getIntersection(Node* headA, Node* headB) {
    while (headA != headB) {
        headA = headA->next;
        headB = headB->next;
    }
    return headA;
}

medium

A. It does not handle different list lengths, causing null pointer errors.

B. It uses incorrect comparison operator for nodes.

C. It returns the wrong node after loop ends.

D. It should use recursion instead of iteration.

Solution

1. Step 1: Analyze pointer movement

   The code moves both pointers forward simultaneously without adjusting for different list lengths.

2. Step 2: Identify consequence

   If lists differ in length, one pointer may reach null before the other, causing a null pointer dereference or infinite loop.

3. Final Answer:

   It does not handle different list lengths, causing null pointer errors. -> Option A

4. Quick Check:

   Unequal lengths cause errors = D [OK]

Hint: Adjust pointers for length difference before comparing [OK]

Common Mistakes:

• Ignoring list length difference
• Dereferencing null pointers
• Assuming simultaneous traversal always works

5. You have two singly linked lists that intersect. List A has length 8, List B has length 5. Which approach correctly finds the intersection point efficiently?

hard

A. Traverse both lists fully and store nodes in hash sets, then find first common node.

B. Calculate length difference, advance longer list by difference, then move both pointers together.

C. Reverse both lists and compare nodes from start until they differ.

D. Compare node values one by one from heads until a matching value is found.

Solution

1. Step 1: Understand length difference method

   Calculate difference (8 - 5 = 3), advance longer list by 3 nodes to align both lists.

2. Step 2: Traverse both lists together

   Move both pointers one step at a time; the first node where they match is the intersection.

3. Final Answer:

   Calculate length difference, advance longer list by difference, then move both pointers together. -> Option B

4. Quick Check:

   Align by length difference then traverse = B [OK]

Hint: Advance longer list by length difference before simultaneous traversal [OK]

Common Mistakes:

• Comparing node values instead of references
• Reversing lists unnecessarily
• Using extra space when not needed