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1473. Find the Longest Substring Containing Vowels in Even Counts

My accepted Python solution to LeetCode problem 1473, Find the Longest Substring Containing Vowels in Even Counts, running in 267ms.

  • Difficulty: Medium
  • Python
  • Runtime 267ms
  • Memory 20.1MB

Read the problem on LeetCode View on GitHub

The problem statement is LeetCode’s and stays on their site. What follows is my accepted solution.

Python

Accepted on LeetCode — runtime 267ms, memory 20.1MB, accepted 2025-12-29.

python
class Solution:
    def findTheLongestSubstring(self, s: str) -> int:
        # Use bitmask to track parity of vowels
        # 5 vowels = 5 bits: a=0, e=1, i=2, o=3, u=4
        # Time: O(n), Space: O(32) = O(1)
        
        vowels = {'a': 0, 'e': 1, 'i': 2, 'o': 3, 'u': 4}
        
        # first_occurrence[mask] = first index where this mask state occurs
        # -1 means mask hasn't occurred (but mask=0 starts at index -1)
        first_occurrence = {0: -1}
        
        mask = 0
        max_len = 0
        
        for i, char in enumerate(s):
            if char in vowels:
                mask ^= (1 << vowels[char])
            
            if mask in first_occurrence:
                max_len = max(max_len, i - first_occurrence[mask])
            else:
                first_occurrence[mask] = i
        
        return max_len

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