SIG (Susquehanna)·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

SIG coding round, one algorithmic problem focused on subarray counting with a zero-parity twist. Pretty clean problem once you see the pattern, but getting there under pressure is a different story.

Questions Asked (1)

Given an integer array, how many contiguous subarrays contain an odd number of zeros? Solve it in O(n) time and explain your approach and complexity.

Algorithms & Data Structures

Author's notes

I stared at the example for longer than I should have.

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AI HintsAI Generated

Suggested Approach

Use a prefix parity approach: track the parity of the number of zeros seen so far and count how many previous prefixes have the opposite parity. This yields an O(n) time, O(1) space solution by maintaining counts of even and odd parity prefixes.

Pro tip: Start by clarifying that 'odd number of zeros' means the count of zeros in the subarray is odd, not the sum of elements. Then mention that the same technique works for any condition based on parity of a count.

1. Clarify the problem

Confirm that we need to count contiguous subarrays where the number of zeros is odd. Ensure the array can contain any integers, and zeros are the only elements that affect the count.

2. Define prefix parity

Let P[i] be the parity (0 for even, 1 for odd) of the number of zeros in the first i elements. A subarray from j+1 to i has odd zeros iff P[i] != P[j].

3. Count valid subarrays

Initialize counts of even and odd parity prefixes. Start with even count = 1 (empty prefix). Iterate through the array, update parity when encountering a zero, and add the count of opposite parity to the answer.

4. Update counts

After processing each element, increment the count for the current parity. This ensures future subarrays can use this prefix.

5. Analyze complexity

Time complexity is O(n) because we traverse the array once. Space complexity is O(1) since we only store two counters and the current parity.

Key Points to Mention

Prefix parity concept: parity of zeros in prefix determines subarray parity.
Condition for odd zeros: prefix parities must differ.
Maintain counts of even and odd parity prefixes.
Initialize even count to 1 for the empty prefix.
Update parity only when encountering a zero.
O(n) time and O(1) space complexity.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.