Write a C# method that returns the first character that appears exactly once in a string.

• Case-sensitive comparison is required.
• Whitespace is a character unless the interviewer changes the rule.
• Empty or null input returns null.
• The answer is the first unique character in original order.

I would make two passes. The first pass counts every character. The second pass walks the original string and returns the first character whose count is one. That keeps the code simple and avoids tricky state. If memory were constrained or the character set were fixed, I could discuss arrays, but a dictionary is the clean general solution.

public static char? FirstUniqueChar(string? text)
{
    if (string.IsNullOrEmpty(text))
    {
        return null;
    }

    var counts = new Dictionary<char, int>();
    foreach (var ch in text)
    {
        counts[ch] = counts.TryGetValue(ch, out var count) ? count + 1 : 1;
    }

    foreach (var ch in text)
    {
        if (counts[ch] == 1)
        {
            return ch;
        }
    }

    return null;
}

Time is O(n) for two linear passes. Space is O(k), where k is the number of distinct characters.

• Null or empty string.
• All characters repeat.
• First character is unique.
• Only one character.
• Case-sensitive examples such as aA.

• Use a string where the first distinct key is not the first unique character.
• Use repeated characters before the unique one.
• Use Unicode or whitespace if the problem statement does not restrict characters.

• Returning the first dictionary key with count one.
• Doing nested scans and turning the solution into O(n^2).
• Changing case when the problem says case-sensitive.