Google·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Google SWE coding round with a single algorithm question about adding two large numbers represented as digit lists. Pretty standard stuff but the edge cases are where they really watch you.

Questions Asked (1)

Given two lists where each element is a digit of a large integer (most significant digit first), write a function that adds the two numbers and returns the result as a list. The lists may be empty or contain only zeros.

Algorithms & Data Structures

Author's notes

The core logic clicked fast for me, reverse both lists, add digit by digit with a carry, reverse the result back.

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AI HintsAI Generated

Suggested Approach

Clarify edge cases first, then propose a two-pointer approach from the least significant digit, handling carry and unequal lengths. Discuss time/space complexity and consider follow-ups like in-place modification or handling negative numbers.

Pro tip: Mention that you would ask about input constraints (e.g., max length, digit range) and whether the result should be a new list or can modify one input. This shows you think about production code, not just the algorithm.

1. Clarify requirements and edge cases

Ask about input size, digit range, whether lists can be empty, and if the result should be a new list. Confirm that digits are 0-9 and no leading zeros except for zero itself.

2. Choose an approach

Propose iterating from the end of both lists (least significant digit) using two pointers, summing digits plus carry, and building the result in reverse. Alternatively, reverse the lists first for simpler indexing.

3. Walk through the algorithm

Explain step-by-step: initialize carry=0, pointers at last indices, loop while pointers valid or carry>0, compute sum, append sum%10 to result, update carry. Handle remaining digits and final carry.

4. Analyze complexity and edge cases

State time complexity O(max(n,m)) and space O(max(n,m)) for the result. Discuss edge cases: empty lists, all zeros, different lengths, carry propagation (e.g., 999+1=1000).

5. Consider optimizations and follow-ups

Mention potential optimizations like in-place modification if allowed, or using a stack. Be prepared for follow-ups: subtract two numbers, handle negative numbers, or multiply.

Key Points to Mention

Two-pointer technique from least significant digit
Carry propagation and handling final carry
Time and space complexity analysis
Edge cases: empty lists, zeros, unequal lengths
Clarifying questions about input constraints and output format
Potential follow-up: in-place modification or negative numbers

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