Longest Ordered Subsequence of Vowels

Problem Given a string of only vowels, find the length of the longest subsequence consisting of all five vowels in order: one or more 'a's, then 'e's, then 'i's, then 'o's, then 'u's.

Input / Output

• Input: string s of characters {a,e,i,o,u}.
• Output: length of the longest valid subsequence (0 if none uses all five).

Constraints

• |s| up to 10^5 — O(n * 5) DP expected; naive memoization on (index, lastChar) without compaction can TLE.

Example

• s = "aeiaaioooaauuaeiou" → 10 (pick six a's, then e, i, o, u in order).