java: add «1636. Sort Array by Increasing Frequency»

Signed-off-by: Matej Focko <me@mfocko.xyz>

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

@@ -0,0 +1,35 @@
+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;
+  }
+}