Decrypt String from Alphabet to Integer Mapping LeetCode solution in Rust

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impl Solution {
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  pub fn freq_alphabets(s: String) -> String {
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    let mut iter = s.chars().rev().peekable();
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    let mut res = String::new();
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    loop {
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      match iter.next() {
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        Some('#') => {
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          let c1 = iter.next().unwrap().to_digit(10).unwrap() as u8;
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          let c2 = iter.next().unwrap().to_digit(10).unwrap() as u8;
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          res.push((c2 * 10 + c1 - 1 + 'a' as u8) as char)
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        }
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        Some(x) => {
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          let c = x.to_digit(10).unwrap() as u8;
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          res.push((c - 1 + 'a' as u8) as char);
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        }
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        _ => {}
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      }
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      if iter.peek() == None {
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        break;
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      }
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    }
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    res.chars().rev().collect()
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  }
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}