Rabbits in forest

/*
 * There is a forest with an unknown number of rabbits. We asked n rabbits "How many rabbits have the same color as you?" and collected the answers in an integer array answers where answers[i] is the answer of the ith rabbit.
 * Given the array answers, return the minimum number of rabbits that could be in the forest.
 * Example 1:
 * Input: answers = [1,1,2]
 * Output: 5
 * Explanation:
 * The two rabbits that answered "1" could both be the same color, say red.
 * The rabbit that answered "2" can't be red or the answers would be inconsistent.
 * Say the rabbit that answered "2" was blue.
 * Then there should be 2 other blue rabbits in the forest that didn't answer into the array.
 * The smallest possible number of rabbits in the forest is therefore 5: 3 that answered plus 2 that didn't.
 * Example 2:
 * Input: answers = [10,10,10]
 * Output: 11
 * Constraints:
 * 1 <= answers.length <= 1000
 * 0 <= answers[i] < 1000
 */
use std::collections::HashMap;

impl Solution {
  pub fn num_rabbits(answers: Vec<i32>) -> i32 {
    let mut freq: HashMap<i32, i32> = HashMap::new();
    for a in answers { *freq.entry(a).or_insert(0) += 1; }
    freq.iter().map(|(&a, &count)| {
      let grp = a + 1;
      ((count + grp - 1) / grp) * grp
    }).sum()
  }
}