Longest Arithmetic Subsequence of Given Difference LeetCode solution in Rust

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const RANGE_MIN: i32 = -10_000;
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const RANGE_MAX: i32 = 10_000;
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impl Solution {
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  pub fn longest_subsequence(arr: Vec<i32>, difference: i32) -> i32 {
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    let range = (RANGE_MAX - RANGE_MIN + 1) as usize;
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    fn offset(i: i32) -> Option<usize> {
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      if RANGE_MIN <= i && i <= RANGE_MAX {
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        Some((i - RANGE_MIN) as usize)
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      } else {
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        None
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      }
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    }
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    /// "Length when last seen"; longest subseq when we last saw that arr[i] value
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    let mut lwls = vec![0_i32; range];
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    let mut best = 1;
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    for &a in arr.iter().rev() {
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      let old = offset(a + difference).map(|i| lwls[i]).unwrap_or(0);
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      let new_idx = offset(a).unwrap(); // safety: problem constraints
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      let len = lwls[new_idx].max(old + 1);
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      lwls[new_idx] = len;
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      best = best.max(len);
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    }
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    best
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  }
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}