Spiral Matrix IV

Spiral Matrix IV

What is this problem about?

The Spiral Matrix IV interview question combines matrix simulation with linked list traversal. You are given the dimensions m x n of a matrix and the head of a linked list. You need to fill the matrix with the values from the linked list in a spiral order. If the linked list ends before the matrix is full, the remaining cells should be filled with -1. This is a multi-data-structure problem that tests your ability to synchronize different traversal methods.

Why is this asked in interviews?

This Spiral Matrix IV coding problem is often used by Microsoft and Google to see if a candidate can handle two different pointers: one for the matrix boundaries and one for the linked list nodes. It requires careful coordination—as you move through the matrix in a spiral, you must also advance through the linked list, checking for the null terminator at each step.

Algorithmic pattern used

This problem uses the Linked List traversal pattern merged with Matrix Simulation. You initialize an m x n matrix filled with -1s. Then, you apply the standard four-boundary spiral logic. For each cell you visit in the spiral, you take the current node's value, place it in the matrix, and move to the next node. If the next node is null, you stop filling and let the remaining -1s stay as they are.

Example explanation

Grid: 3x3, Linked List: 3 -> 0 -> 2 -> 6 -> 8.

1. Top row: (0,0)=3, (0,1)=0, (0,2)=2.
2. Right column: (1,2)=6, (2,2)=8.
3. The linked list is now empty.
4. Remaining cells (2,1), (2,0), (1,0), (1,1) remain as -1. Result:

[[3, 0, 2],
 [-1, -1, 6],
 [-1, -1, 8]]

(Note: the spiral continues into (2,1), (2,0), etc., but the list is empty).

Common mistakes candidates make

• Null Pointer Exceptions: Forgetting to check if the linked list node is null before trying to access its value.
• Matrix Initialization: Failing to pre-fill the matrix with -1, requiring extra logic later to fill gaps.
• Boundary Management: The same errors as in Spiral Matrix I—overshooting rows or columns.
• Incorrect Order: Filling the matrix in a standard row-by-row manner instead of the requested spiral order.

Interview preparation tip

When dealing with multiple data structures, think about which one "drives" the loop. In this case, the matrix spiral order drives the movement, while the linked list provides the values. Always ensure your "consumer" (the matrix filler) gracefully handles the case where the "provider" (the linked list) runs dry.