Check If Word Is Valid After Substitutions LeetCode solution in Rust

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impl Solution {
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  pub fn is_valid(s: String) -> bool {
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    let mut stack: Vec<[u8; 3]> = vec![[0, 0, 0]; 1];
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    for byte in s.as_bytes() {
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      let idx = (*byte - b'a') as usize;
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      if stack.last().unwrap()[idx] == 1 {
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        stack.push([0, 0, 0]);
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      }
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      let counts = stack.last_mut().unwrap();
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      counts[idx] = 1;
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      if counts[2] > counts[1] || counts[2] > counts[0] || counts[1] > counts[0] {
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        return false;
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      } else if *counts == [1, 1, 1] {
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        stack.pop();
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        if stack.len() == 0 {
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          stack.push([0, 0, 0]);
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        }
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      }
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    }
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    stack.len() == 1 && stack[0] == [0, 0, 0]
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  }
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}