Largest Substring Between Two Equal Characters LeetCode solution in Rust

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use std::cmp::max;
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impl Solution {
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  pub fn max_length_between_equal_characters(s: String) -> i32 {
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    let mut firsts = [None; 26];
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    let mut max_len = -1;
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    for (i, char) in s.chars().enumerate() {
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      let i = i as i32;
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      let j = (char as u8 - b'a') as usize;
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      match firsts[j] {
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        Some(first) => {
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          max_len = max(i - first, max_len);
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        }
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        None => {
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          firsts[j] = Some(i + 1);
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        }
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      }
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    }
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    max_len
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  }
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}