Search for a Value in Linked List in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a linked list?

A linked list is a chain of nodes where each node holds data and a link to the next node. It is like a treasure hunt where each clue leads to the next one.

beginner

How do you search for a value in a linked list?

Start at the first node and check its value. If it matches, stop. If not, move to the next node. Repeat until you find the value or reach the end.

beginner

What happens if the value is not found in the linked list?

You reach the end of the list (null) without finding the value. This means the value is not in the list.

intermediate

Why is searching in a linked list slower than in an array?

Because you must check nodes one by one from the start. Unlike arrays, you cannot jump directly to the middle.

intermediate

What is the time complexity of searching a value in a linked list?

The time complexity is O(n), where n is the number of nodes, because you may need to check each node once.

What do you check first when searching for a value in a linked list?

A The value of the current node

B The value of the last node

C The size of the list

D The middle node

You always start by checking the value of the current node, beginning with the head.

If the value is not found, what will the search function return?

A Pointer to the last node

B The first node

C Null or equivalent

D The middle node

If the value is not found, the search reaches the end and returns null (no node found).

What is the best case time complexity for searching a value in a linked list?

A O(n^2)

B O(n)

C O(log n)

D O(1)

If the value is in the first node, you find it immediately, so best case is O(1).

Why can't you jump directly to the middle node in a linked list?

A Because nodes are not stored in continuous memory

B Because linked lists are sorted

C Because nodes have no values

D Because linked lists have only one node

Linked list nodes are scattered in memory and linked by pointers, so you must follow links one by one.

What is the main pointer used to start searching in a linked list?

A Tail pointer

B Head pointer

C Middle pointer

D Random pointer

The head pointer points to the first node and is where searching begins.

Explain step-by-step how to search for a value in a linked list.

Think of walking through a chain checking each link.

Describe why searching in a linked list takes longer than searching in an array.

Compare walking through a chain vs. jumping to a shelf number.

Practice

1. What is the main purpose of searching for a value in a linked list?

easy

A. To add a new node at the end of the list

B. To find if a specific value exists in the list

C. To delete the first node of the list

D. To reverse the order of nodes in the list

Solution

1. Step 1: Understand the search operation

   Searching in a linked list means checking each node to see if it contains the value we want.

2. Step 2: Identify the goal of searching

   The goal is to find whether the value exists anywhere in the list, not to modify the list.

3. Final Answer:

   To find if a specific value exists in the list -> Option B

4. Quick Check:

   Search means find value presence = To find if a specific value exists in the list [OK]

Hint: Search means check each node for the value [OK]

Common Mistakes:

• Confusing search with insertion or deletion
• Thinking search changes the list
• Assuming search reverses the list

2. Which of the following is the correct syntax to start traversing a singly linked list in C++?

easy

A. Node* current = new Node();

B. Node current = head;

C. Node current = new Node();

D. Node* current = head;

Solution

1. Step 1: Understand pointer usage in linked lists

   In C++, linked list nodes are accessed via pointers, so we use a pointer variable to traverse.

2. Step 2: Identify correct pointer initialization

   We set a pointer 'current' to point to the head node to start traversal: Node* current = head;

3. Final Answer:

   Node* current = head; -> Option D

4. Quick Check:

   Use pointer to start traversal = Node* current = head; [OK]

Hint: Use pointer to traverse linked list starting at head [OK]

Common Mistakes:

• Using object instead of pointer for traversal
• Creating a new node instead of using existing head
• Not initializing traversal pointer correctly

3. What will be the output of the following code snippet?

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

Node* head = new Node{10, nullptr};
head->next = new Node{20, nullptr};
head->next->next = new Node{30, nullptr};

Node* current = head;
int searchValue = 20;
bool found = false;
while (current != nullptr) {
  if (current->data == searchValue) {
    found = true;
    break;
  }
  current = current->next;
}
if (found) std::cout << "Value found";
else std::cout << "Value not found";

medium

A. Value not found

B. Compilation error

C. Value found

D. Runtime error

Solution

1. Step 1: Trace the linked list nodes and values

   The list has nodes with values 10 -> 20 -> 30. We search for 20.

2. Step 2: Check the search loop logic

   The loop checks each node's data. When it reaches node with 20, found becomes true and loop breaks.

3. Final Answer:

   Value found -> Option C

4. Quick Check:

   20 exists in list, so output = "Value found" [OK]

Hint: Check each node's data until match or end [OK]

Common Mistakes:

• Forgetting to update current pointer
• Using wrong comparison operator
• Not breaking loop after finding value

4. Identify the error in the following code that searches for a value in a linked list:

Node* current = head;
int searchValue = 15;
while (current->next != nullptr) {
  if (current->data == searchValue) {
    std::cout << "Found";
    break;
  }
  current = current->next;
}
if (current == nullptr) std::cout << "Not Found";

medium

A. The loop condition skips checking the last node

B. The searchValue variable is not initialized

C. The current pointer is not updated inside the loop

D. The output statements are missing semicolons

Solution

1. Step 1: Analyze the loop condition

   The loop runs while current->next is not nullptr, so it stops before the last node.

2. Step 2: Understand impact on search

   The last node's data is never checked, so if the value is there, it will be missed.

3. Final Answer:

   The loop condition skips checking the last node -> Option A

4. Quick Check:

   Loop must check all nodes including last [OK]

Hint: Loop on current != nullptr to check all nodes [OK]

Common Mistakes:

• Using current->next != nullptr instead of current != nullptr
• Not checking last node's data
• Incorrectly handling nullptr checks

5. You want to search for a value in a singly linked list and return its position (1-based index). Which approach correctly implements this?

int searchPosition(Node* head, int value) {
  int pos = 1;
  Node* current = head;
  while (current != nullptr) {
    if (current->data == value) return pos;
    current = current->next;
    pos++;
  }
  return -1;
}

hard

A. Returns the position of value or -1 if not found

B. Returns true if value found, false otherwise

C. Returns the last node's data always

D. Returns 0 if value is found at head

Solution

1. Step 1: Understand the function logic

   The function starts position at 1 and traverses nodes, checking data against value.

2. Step 2: Check return values

   If value matches, it returns current position; if not found, returns -1.

3. Final Answer:

   Returns the position of value or -1 if not found -> Option A

4. Quick Check:

   Function returns index or -1 if missing [OK]

Hint: Return index on match, else -1 after full traversal [OK]

Common Mistakes:

• Returning boolean instead of position
• Starting position count at 0 instead of 1
• Not returning -1 when value not found