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+package theweeklychallenge;
+
+/*
+
+Week 174:
+
+    https://theweeklychallenge.org/blog/perl-weekly-challenge-174
+
+Task #2: Permutation Ranking
+
+    You are given a list of integers with no duplicates, e.g. [0, 1, 2].
+
+    Write two functions, permutation2rank() which will take the list
+    and determine its rank (starting at 0) in the set of possible
+    permutations arranged in lexicographic order, and rank2permutation()
+    which will take the list and a rank number and produce just that
+    permutation.
+
+Compile and Run:
+
+    mohammad-anwar/java$ javac theweeklychallenge/PermutationRanking.java
+    mohammad-anwar/java$ java  theweeklychallenge.PermutationRanking
+
+*/
+
+import java.util.List;
+import java.util.Arrays;
+import java.util.ArrayList;
+import java.util.stream.Collectors;
+import junit.framework.TestCase;
+import static junit.framework.Assert.*;
+
+public class PermutationRanking extends TestCase {
+
+    public static void main(String[] args) {
+        junit.textui.TestRunner.run(
+            theweeklychallenge.PermutationRanking.class);
+    }
+
+    public void test_permutation2rank() {
+        int[]                 n = {1, 0, 2};
+        List<List<Integer>> got = permutation2rank(n);
+        List<List<Integer>> exp = List.of(
+            List.of(0, 1, 2),
+            List.of(0, 2, 1),
+            List.of(1, 0, 2),
+            List.of(1, 2, 0),
+            List.of(2, 0, 1),
+            List.of(2, 1, 0)
+        );
+        assertEquals(exp, got);
+    }
+
+    public void test_rank2permutation() {
+        int[]       n = {1, 0, 2};
+        Integer[] got = rank2permutation(n, 1);
+        Integer[] exp = {0, 2, 1};
+        assertEquals(Arrays.toString(exp), Arrays.toString(got));
+    }
+
+    public static List<List<Integer>> permutation2rank(int[] n) {
+        List<List<Integer>> perm = new ArrayList<>();
+        Permutation(0, n, perm);
+        return sortPermutations(perm);
+    }
+
+    public static Integer[] rank2permutation(int[] n, int r) {
+        List<List<Integer>> rank = permutation2rank(n);
+        List<Integer> p = rank.get(r);
+        return p.toArray(new Integer[p.size()]);
+    }
+
+    private static List<List<Integer>> sortPermutations(List<List<Integer>> perm) {
+        String[] strPerm = new String[perm.size()];
+        int i = 0;
+        for(List<Integer> _perm : perm) {
+            strPerm[i] = _perm.stream()
+                              .map(String::valueOf)
+                              .collect(Collectors.joining(""));
+            i++;
+        }
+
+        Arrays.sort(strPerm);
+
+        List<List<Integer>> sortedPerm = new ArrayList<>();
+        for(int j = 0; j < strPerm.length; j++) {
+            String        str = strPerm[j];
+            String[] strArray = str.split("");
+            int[]    intArray = new int[strArray.length];
+
+            for(int k = 0; k < strArray.length; k++) {
+                intArray[k] = Integer.parseInt(strArray[k]);
+            }
+
+            List<Integer> intList = new ArrayList<Integer>(intArray.length);
+            for(int e : intArray) {
+                intList.add(e);
+            }
+
+            sortedPerm.add(intList);
+        }
+
+        return sortedPerm;
+    }
+
+    // https://www.w3resource.com/java-exercises/array/java-array-exercise-68.php
+    private static void Permutation(int i, int[] nums, List<List<Integer>> result) {
+        if (i == nums.length - 1) {
+            List<Integer> list = new ArrayList<>();
+            for(int n : nums) list.add(n);
+            result.add(list);
+        } else {
+            for(int j = i, l = nums.length; j < l; j++) {
+                int temp = nums[j];
+                nums[j]  = nums[i];
+                nums[i]  = temp;
+                Permutation(i + 1, nums, result);
+                temp     = nums[j];
+                nums[j]  = nums[i];
+                nums[i]  = temp;
+            }
+        }
+    }
+}