Longest Substring with At Most Two of Each Character

# Longest Substring with At Most Two of Each Character Given a string `s`, return the maximum length of a contiguous substring in which no character occurs more than twice. The limit applies separately to each character. A valid substring may contain more than two distinct characters. ### Examples **Example 1** ```text Input: s = "abcabcbb" Output: 6 ``` The first six characters, `"abcabc"`, contain two occurrences each of `a`, `b`, and `c`. The two length-seven substrings each contain at least three occurrences of `b`, so neither they nor the whole string are valid. **Example 2** ```text Input: s = "abaccca" Output: 5 ``` The substring `"abacc"` is valid and has length five. Both length-six substrings contain three occurrences of `c`, and the whole string is also invalid, so a longer valid substring does not exist. ### Constraints - `2 <= s.length <= 100`. - `s` contains only lowercase English letters. ### Clarifying Questions - Is this a subsequence problem? No; the chosen characters must be contiguous. - Does every present character have to occur exactly twice? No; one or two occurrences are allowed. - Is there a limit of two distinct characters? No; each distinct character has its own two-occurrence limit. - Is the substring itself returned? No; return only its length. ### C++ Interface ```cpp class Solution { public: int maximumLengthSubstring(string s) { } }; ``` ### Reasoning Checkpoints - Apply the occurrence limit independently to every character in a contiguous interval. - Establish that the returned length is achievable and no longer valid interval exists. - Cover repeated characters and the input-length boundaries, and justify resource use. ```hint Enumerate starts and extend the end The input has at most 100 characters. For each starting position, extend a substring while maintaining character counts. Once one count exceeds two, further extension from that same start cannot make it valid again. ```