Salesforce·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Salesforce coding interview with a tricky string simulation problem. The naive approach works on small inputs but falls apart fast, and the interviewer clearly expected you to know that going in.

Questions Asked (1)

You have a binary string. Every second, all occurrences of '01' in the string simultaneously become '10'. This repeats until no '01' exists. How many seconds does the process take to complete?

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Started with the obvious simulation and coded it up cleanly, then they asked about performance on a long string and I kind of froze.

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AI HintsAI Generated

Suggested Approach

Model the process as each '1' moving right past adjacent '0's, where the total time equals the maximum over all '1's of the number of '0's to its right that have no '1' between them. Derive a formula by scanning the string and tracking the count of zeros after the last one, updating the answer as the maximum of (zeros + ones) for each '1' encountered. Explain the reasoning clearly and provide a linear-time algorithm.

Pro tip: After presenting the solution, mention that the same logic can be applied to similar problems like counting the number of swaps to group all 1s together, showing pattern recognition and transferable problem-solving skills.

1. Understand the process

Clarify that each second, every '01' becomes '10' simultaneously, meaning each '1' can swap with a '0' immediately to its right, but only one swap per second per '1'.

2. Identify the key observation

Recognize that the total time is determined by the '1' that takes the longest to move past all zeros to its right, which depends on the number of zeros after it and the number of ones before it.

3. Derive the formula

Scan left to right, maintaining a count of zeros seen so far and ones seen so far. For each '1', the time for it to pass all zeros to its right is (zeros seen so far + ones seen so far). Update the maximum time.

4. Implement and test

Write a linear-time algorithm that computes the maximum, and test with examples like '0011' (0 seconds), '1010' (1 second), '1001' (2 seconds) to verify correctness.

5. Discuss complexity and edge cases

State that the solution runs in O(n) time and O(1) space, and handle edge cases like empty string or all same characters.

Key Points to Mention

Simultaneous swaps mean each '1' moves right by at most one position per second.
The total time is the maximum over all '1's of (number of zeros to its right + number of ones to its left).
Linear scan with two counters (zeros and ones) yields O(n) time and O(1) space.
The process is equivalent to sorting the binary string in non-decreasing order (all 0s then all 1s).
Edge cases: already sorted string takes 0 seconds; string with no '01' takes 0 seconds.
The problem can be related to counting inversions or bubble sort passes.

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