Start by clarifying the problem: define 'non-repeating characters' (characters that appear exactly once) and discuss constraints like case sensitivity, character set (ASCII vs Unicode), and expected output format. Then propose an efficient solution using a hash map or array to count frequencies in one pass, followed by a second pass to collect characters with count 1, analyzing time and space complexity.
Pro tip: Mention that for ASCII strings, a fixed-size array of 256 integers is more efficient than a hash map, and always discuss trade-offs between time and space. Also, consider edge cases like empty string, all repeating characters, and Unicode to show thoroughness.
Ask if 'non-repeating' means appearing exactly once, and confirm case sensitivity, character set (ASCII/Unicode), and output format (e.g., list of characters or their indices).
Select a hash map (dictionary) for general character sets or a fixed-size array for ASCII to count frequencies efficiently.
Iterate through the string once, incrementing the count for each character in the chosen data structure.
Iterate through the string again (or through the data structure) to gather characters with a count of exactly 1, preserving order if needed.
State time complexity O(n) and space complexity O(k) where k is the number of unique characters, and discuss handling of empty strings, all repeating characters, and Unicode.
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