3104. Find Longest Self-Contained Substring

Fix the start index of the substring.

For some fixed index start, let's try to find some index end such that this substring satisfies the property and also end is as maximum as possible.

Write some recursive function shrink(start, end) that gives a substring. If the substring is valid, then return end. Otherwise, it reduces <end to reach some valid end.

For some shrink(start, end), if the substring is not valid, it means there is some character that is both inside and outside of the substring. Now try to reduce end such that it does not contain that character anymore.

If you implement the shrink(start, end) function optimally, you'll achieve O(n * 26 * 26) by using partial sum.