Split Linked List in Parts

Split Linked List in Parts

What is this problem about?

The Split Linked List in Parts coding problem asks you to divide a single linked list into $k$ consecutive parts. The goal is to make the parts as equal in size as possible. If the total number of nodes $N$ is not perfectly divisible by $k$, the first few parts should have one extra node compared to the later parts. For example, if you have 10 nodes and $k=3$, the parts should have sizes 4, 3, and 3. If $k$ is greater than the number of nodes, some parts will be null.

Why is this asked in interviews?

This is a standard Linked List interview pattern question frequently asked by Microsoft, Meta, and Amazon. It tests your basic data structure manipulation skills, specifically your ability to traverse a list, calculate its length, and correctly "cut" the connections between nodes. It also checks your ability to handle basic arithmetic like integer division and remainders in a practical context.

Algorithmic pattern used

The algorithm is a straightforward two-pass approach. First, traverse the entire list to find the total number of nodes $N$. Second, calculate the base size of each part (N // k) and the number of parts that need an extra node (N % k). Then, traverse the list again, counting out the required number of nodes for each part and setting the next pointer of the last node in each part to null to separate them.

Example explanation (use your own example)

Imagine a list [1, 2, 3, 4, 5] and $k = 3$.

1. Length $N = 5$.
2. Base size = $5 // 3 = 1$. Remainder = $5 \% 3 = 2$.
3. Part 1 needs $1 + 1 = 2$ nodes: [1, 2].
4. Part 2 needs $1 + 1 = 2$ nodes: [3, 4].
5. Part 3 needs 1 node: [5]. The result is an array containing the heads of these three lists: [[1, 2], [3, 4], [5]].

Common mistakes candidates make

• Losing the head: Forgetting to store the head of each new part before cutting the previous part's tail.
• Null pointer exceptions: Not checking if the current node is null when $k > N$.
• Off-by-one errors: Incorrectly calculating how many parts should have the "extra" node.
• Memory leaks: In languages like C++, not being careful about how the list is partitioned if ownership isn't clear (though usually not an issue in standard competitive programming environments).

Interview preparation tip

For linked list problems, always dry run your code with a very small example (like $N=2, k=3$ or $N=5, k=2$) to ensure your pointer logic is correct. Being able to explain the "cut" operation (setting node.next = null) clearly is crucial for showing you understand how linked structures work.