Find the length of Longest Substring

A longest-substring prompt is not complete until the property is named: unique characters, at most k distinct characters, balanced parentheses, a dictionary rule, or something else. Ask for that rule, the character model, and whether the interviewer truly means contiguous substring rather than subsequence. For many monotonic properties, maintain a sliding window with counts or last-seen positions. Expand right, update state, then shrink left only until the window is valid again; its invariant must state exactly why every retained window is eligible. For example, for no repeats, move left past the prior occurrence of the entering character. If the property is not monotonic, such as palindrome length, use an appropriate different method rather than forcing a window. Test empty string, one character, repeated runs, a best answer at an endpoint, and Unicode handling. Typical windows are O(n) time because each pointer moves at most n times, with O(k) space for tracked characters.