1
impl Solution {
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  pub fn appeal_sum(s: String) -> i64 {
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    let n = s.len();
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    let mut f = vec![0; n];
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    let mut last_pos = [-1; 26];
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    // println!("last_pos {:?}", last_pos);
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    for (i, c) in s.chars().enumerate() {
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      let char_id = c as usize - 'a' as usize;
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      let i2 = i as i64;
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      // println!("char id {}", char_id);
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      if i == 0 {
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        f[i] = 1;
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        last_pos[char_id] = i2;
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        continue;
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      }
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      f[i] += f[i - 1] + i2 - last_pos[char_id];
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      last_pos[char_id] = i2;
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    }
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    let ans = f.iter().sum();
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    ans
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  }
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}