Technical Interview Preparation:

C Programming Series: Solving the Linked List Cycle Problem

In a technical interview, you may encounter a question that requires you to solve a programming problem on the spot. It can be nerve-wracking, especially if you’re not prepared. That’s why it’s essential to practice and sharpen your skills beforehand. One of the problems that might be given is to check if a singly linked list has a cycle in it. In this article, we’ll discuss how to approach and solve this problem in C. The problem can be found in this repository on GitHub.

Problem Description

The problem asks us to write a function that takes a singly linked list as input and determines whether it has a cycle. In other words, the function should return 1 if the linked list contains a cycle and 0 if it does not.

Approach

To solve this problem, we can use the “Floyd’s cycle-finding algorithm,” also known as the “tortoise and hare algorithm.” The idea behind this algorithm is to use two pointers, one moving faster than the other. If there is a cycle, the faster pointer (the hare) will eventually catch up with the slower pointer (the tortoise).

Let’s apply this algorithm step by step:

Step 1: Initialize two pointers, slow and fast, both pointing to the head of the linked list.

int check_cycle(listint_t *list)
{
    listint_t *slow = list;
    listint_t *fast = list;
}

Step 2: Move the fast pointer by two nodes and the slow pointer by one node.

int check_cycle(listint_t *list)
{
    listint_t *slow = list;
    listint_t *fast = list;
    while (fast != NULL && fast->next != NULL)
    {
        slow = slow->next;
        fast = fast->next->next;
    }
}

Step 3: Check if there is a cycle by comparing the addresses of the two pointers. If they are equal, it means there is a cycle, and we can return 1. Otherwise, we continue with the loop until the fast pointer reaches the end of the list.

int check_cycle(listint_t *list)
{
    listint_t *slow = list;
    listint_t *fast = list;
    while (fast != NULL && fast->next != NULL)
    {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast)
        {
            return 1;
        }
    }

    return 0;
}

Solution Explanation

The solution initializes two pointers, slow and fast, both pointing to the head of the linked list. It then uses a while loop to iterate through the linked list. In each iteration, the slow pointer moves by one node, while the fast pointer moves by two nodes. If there is a cycle, the fast pointer will eventually catch up with the slow pointer, and we can return 1 to indicate that there is a cycle. If the loop finishes without finding a cycle, we can return 0 to indicate that there is no cycle.

Note that we use the != operator to check if the pointers are not NULL. We also use the -> operator to access the next node in the linked list.

The problem requirements specify that we can only use the following functions: write, printf, putchar, puts, malloc, free. Since we don’t need any of these functions to solve the problem, we didn’t use them in our solution.

Efficiency Analysis

The time complexity of this algorithm is O(n), where n is the number of nodes in the linked list. The space complexity is O(1), as we are only using two pointers.

Conclusion

In this article, we discussed how to approach and solve the problem of checking if a singly linked list has a cycle in it using C programming language. We used the Floyd’s cycle-finding algorithm that involves using two pointers — one moving faster than the other — to determine if a cycle exists in the linked list. Our solution had a time complexity of O(n) and a space complexity of O(1).

Preparing for technical interviews can be challenging, but with practice and proper guidance, it becomes easier. It’s essential to remember to practice coding problems daily, understand the concepts behind the algorithms, and sharpen your problem-solving skills. Additionally, it’s good to familiarize yourself with the tools and languages that you will be using during the interview. Lastly, always approach the problem with a clear and systematic mind and communicate your thought process effectively.

In conclusion, technical interviews can be nerve-wracking, but with adequate preparation, you can increase your chances of succeeding. The linked list cycle problem we solved is just one of the many questions you may encounter. So, keep practicing, and best of luck in your technical interview preparation!