Number of unique flavors after sharing k candies

use std::collections::{HashMap, HashSet};

impl Solution {
  pub fn share_candies(candies: Vec<i32>, k: i32) -> i32 {
    let k = k as usize;
    let n = candies.len();
    if k == 0 {
      let unique: HashSet<i32> = candies.iter().cloned().collect();
      return unique.len() as i32;
    }

    // Count total frequency
    let mut total: HashMap<i32, usize> = HashMap::new();
    for &c in &candies {
      *total.entry(c).or_insert(0) += 1;
    }

    // Initial window: first k elements given away
    let mut window: HashMap<i32, usize> = HashMap::new();
    for i in 0..k {
      *window.entry(candies[i]).or_insert(0) += 1;
    }

    // Count unique flavors kept (those with total[c] - window[c] > 0)
    let mut unique = 0i32;
    for (&c, &t) in &total {
      let w = window.get(&c).copied().unwrap_or(0);
      if t > w {
        unique += 1;
      }
    }

    let mut ans = unique;

    // Slide window
    for i in k..n {
      let out = candies[i - k];
      let entering = candies[i];

      // Remove out from window
      let w_out = window.get(&out).copied().unwrap_or(0);
      let t_out = *total.get(&out).unwrap();
      if t_out - w_out == 0 {
        // was not kept, now will be kept
        unique += 1;
      }
      *window.entry(out).or_insert(0) -= 1;

      // Add entering to window
      let w_in = window.get(&entering).copied().unwrap_or(0);
      let t_in = *total.get(&entering).unwrap();
      if t_in - w_in == 1 {
        // was kept with count 1, now won't be kept
        unique -= 1;
      }
      *window.entry(entering).or_insert(0) += 1;

      ans = ans.max(unique);
    }

    ans
  }
}