Two passes: first to build a frequency count, second to scan for the first count-of-one.
Clarify the problem constraints (e.g., character set, case sensitivity) and then propose an efficient solution using a hash map to count character frequencies in one pass, followed by a second pass to find the first character with count 1. Discuss time and space complexity, and consider edge cases like empty string or all repeating characters.
Pro tip: Mention that for ASCII strings, you can use a fixed-size array of 256 integers instead of a hash map for O(1) space and faster access, but be prepared to discuss trade-offs for Unicode. Also, explicitly state that the two-pass approach is optimal because it preserves the original order while achieving O(n) time.
Ask about the character set (ASCII vs Unicode), case sensitivity, and whether the string can be empty. This shows attention to detail and avoids incorrect assumptions.
Explain that you will use a frequency map (hash map or array) to count occurrences of each character in the first pass, then iterate through the string again to find the first character with a count of 1.
State that the time complexity is O(n) and space complexity is O(k) where k is the number of unique characters (or O(1) for fixed ASCII). Mention that this is optimal for the problem.
Discuss what happens for empty strings, strings with all repeating characters, and strings with special characters. Ensure the solution returns -1 when no unique character exists.
Write clean code with meaningful variable names, and walk through a test case (e.g., 'boeing' returns 0 for 'b', or 'stress' returns 2 for 'e') to verify correctness.
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