Find the longest substring containing vowels in even counts

Hash Table String Bit Manipulation Prefix Sum

75.7% acceptance

Feb 25, 2026

2558

142

Given the string s, return the size of the longest substring containing each vowel an even number of times. That is, 'a', 'e', 'i', 'o', and 'u' must appear an even number of times.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn find_the_longest_substring(s: String) -> i32 {
    // vowels: a=0, e=1, i=2, o=3, u=4
    let vowel_mask = |c: u8| -> u32 {
      match c {
        b'a' => 1, b'e' => 2, b'i' => 4, b'o' => 8, b'u' => 16, _ => 0
      }
    };
    let mut seen = std::collections::HashMap::new();
    seen.insert(0u32, -1i32);
    let mut mask = 0u32;
    let mut ans = 0i32;
    for (i, b) in s.bytes().enumerate() {
      mask ^= vowel_mask(b);
      if let Some(&prev) = seen.get(&mask) {
        ans = ans.max(i as i32 - prev);
      } else {
        seen.insert(mask, i as i32);
      }
    }
    ans
  }
}