How do you find the longest substring without repeating characters?

Short answer: Sliding window with a map/set of characters in the current window. Expand right; when a duplicate appears, shrink left until the window is unique again. Track max window length.

Sample solution

int LengthOfLongestSubstring(string s) {
    var last = new Dictionary<char, int>();
    int left = 0, best = 0;
    for (int right = 0; right < s.Length; right++) {
        char c = s[right];
        if (last.TryGetValue(c, out int prev) && prev >= left)
            left = prev + 1;
        last[c] = right;
        best = Math.Max(best, right - left + 1);
    }
    return best;
}

Complexity

Time O(n), Space O(min(n, alphabet)).

Edge cases to mention

• Empty string
• All unique
• All same character
• Unicode / case sensitivity

Name the pattern “variable-size sliding window” — it signals seniority.