← J.P. Morgan Interview Insights
Pretty clean problem once you see the pattern.
First, clarify that the substrings must be of the form 0^a 1^a or 1^a 0^a, meaning they consist of a single block of 0s followed by a single block of 1s (or vice versa) with equal counts. Then, scan the string to identify maximal runs of identical characters and count valid substrings within each pair of adjacent runs by taking the minimum of their lengths. Alternatively, use a two-pointer sliding window to count all such substrings in O(n) time.
Pro tip: Demonstrate awareness of edge cases (e.g., empty string, all same characters) and discuss time/space complexity trade-offs. Mention that the problem reduces to counting valid substrings within adjacent runs, which can be done in a single pass.
Confirm that substrings must have all 0s grouped and all 1s grouped, and that the groups must be contiguous. Ask if overlapping substrings are counted.
Recognize that valid substrings are exactly those that lie entirely within a concatenation of two adjacent runs of different characters, with equal length from each run.
Choose an approach: either scan runs and sum min(len(run_i), len(run_{i+1})) for each adjacent pair, or use a two-pointer sliding window to count valid substrings directly.
Consider empty string, strings with only one character, and very long runs. Ensure the algorithm handles these efficiently.
State that the solution runs in O(n) time and O(1) extra space, which is optimal for this problem.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Looked easy, and mostly was, but I kept second-guessing the section reset logic.
Clarify the input format and output requirements, then design a single-pass algorithm that tracks chapter and section counters. Use a stack or counters to manage hierarchical numbering, and handle edge cases like missing chapters or multiple sections per chapter.
Pro tip: Mention that you would validate the input and handle edge cases such as sections appearing before any chapter, or lines with extra spaces after the markers. This shows attention to detail and robustness, which is crucial in financial systems.
Ask about input format (e.g., are there other markers? what about empty lines?), output format (e.g., indentation, numbering style), and edge cases (e.g., sections without chapters, multiple chapters).
Propose a single-pass solution using two counters: one for chapters and one for sections. Reset the section counter when a new chapter is encountered.
Iterate through each line, check if it starts with '##' or '#', extract the title, increment the appropriate counter, and format the output string.
Decide how to handle sections before any chapter (e.g., ignore, assign to chapter 0, or throw error) and lines with extra spaces after markers.
Walk through examples, test edge cases, and discuss time/space complexity (O(n) time, O(1) extra space).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.