LeetCode/java/sort-array-by-increasing-frequency.java

import java.util.Comparator;

class Solution {
  private HashMap<Integer, Integer> getFreqs(int[] nums) {
    HashMap<Integer, Integer> freqs = new HashMap<>();

    for (int x : nums) {
      freqs.compute(x, (k, v) -> (v == null) ? 1 : v + 1);
    }

    return freqs;
  }

  public int[] frequencySort(int[] nums) {
    // get the frequencies
    var freqs = getFreqs(nums);

    // convert the array
    Integer[] sortedNums = new Integer[nums.length];
    for (int i = 0; i < nums.length; ++i) {
      sortedNums[i] = nums[i];
    }

    // sort it
    Arrays.sort(
        sortedNums, Comparator.comparing(k -> freqs.get((int) k)).thenComparing(k -> -(int) k));

    // fill back the original array
    for (int i = 0; i < nums.length; ++i) {
      nums[i] = sortedNums[i];
    }

    return nums;
  }
}