Matrix Questions at Anduril: What to Expect

Matrix questions appear in 9 of Anduril's 43 coding problems, making them a significant portion of their technical screen. For a defense technology company building autonomous systems and real-time sensor networks, matrix operations are not academic—they're practical. Processing LIDAR data, managing drone swarm coordinates, simulating sensor grids, and handling image data from surveillance systems all involve 2D arrays. Your ability to traverse, transform, and analyze matrices directly reflects your capacity to work with the core spatial and sensor data structures in their products.

What to Expect — Types of Problems

Anduril's matrix problems typically focus on efficient traversal and state management. Expect variations on these core types:

• Traversal Patterns: Spiral order, diagonal traversal, zigzag (snake) order. These test your ability to systematically navigate a grid while managing boundaries.
• Search and Modification: Searching in a row-wise and column-wise sorted matrix, setting rows/columns to zero in-place, or applying transformations based on neighboring cells. These assess your skill in optimizing for space and time with real-world constraints.
• Simulation and State Change: Problems like the "Game of Life" or flood fill, where the next state depends on adjacent cells. This mirrors simulating sensor updates or agent behaviors in a grid-based environment.
• Pathfinding and Connectivity: Counting islands (connected components) or finding minimum paths in a grid, relevant for navigation and mapping algorithms.

The constraints often emphasize in-place operations and O(1) extra space when possible, reflecting embedded systems or high-performance computing priorities.

How to Prepare — Study Tips with One Code Example

Master a few fundamental patterns rather than memorizing solutions. Practice writing bug-free code by hand on a whiteboard. Focus on:

1. Index manipulation: Use variables like top, bottom, left, right to define boundaries during traversal.
2. Direction arrays: For problems involving 4-directional movement (up, down, left, right), pre-define arrays like [(-1,0), (1,0), (0,-1), (0,1)] to avoid repetitive code.
3. In-place modification: Learn to use the matrix itself to store state, such as using a placeholder value to mark cells for later update.

A key pattern is rotating a square matrix 90 degrees clockwise in-place. This combines index manipulation and in-place swapping.

def rotate(matrix):
    n = len(matrix)
    # Transpose the matrix
    for i in range(n):
        for j in range(i + 1, n):
            matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
    # Reverse each row
    for i in range(n):
        matrix[i].reverse()

The algorithm works by first transposing the matrix (swapping rows and columns) and then reversing each row. This achieves the rotation in O(1) extra space.

Recommended Practice Order

Build competence progressively:

1. Start with basic traversals: print matrix in spiral order, traverse diagonally.
2. Move to search problems: search in a sorted matrix, set matrix zeroes.
3. Tackle simulation: Game of Life, flood fill.
4. Advance to connectivity: number of islands, shortest path in a binary matrix.
5. Finally, practice complex in-place operations: rotate image, reshape matrix.

Always analyze time and space complexity aloud during practice to build interview habits.

Practice Matrix at Anduril