solves permutations

Author: anishLearnsToCode

README.md

@@ -46,6 +46,7 @@
 |  40  | [Combination Sum II](https://leetcode.com/problems/combination-sum-ii)                                                                                    |                                                                [![Java](assets/java.png)](src/CombinationSumII.java)                                                                 |                                                                                                                                   |
 |  43  | [Multiply Strings](https://leetcode.com/problems/multiply-strings)                                                                                        |                                                                 [![Java](assets/java.png)](src/MultiplyStrings.java)                                                                 |                                                                                                                                   |
 |  45  | [Jump Game II](https://leetcode.com/problems/jump-game-ii)                                                                                                |                                                                   [![Java](assets/java.png)](src/JumpGameII.java)                                                                    |                                                                                                                                   |
+|  46  | [Permutations](https://leetcode.com/problems/permutations)                                                                                                |                                                                  [![Java](assets/java.png)](src/Permutations.java)                                                                   |                                                                                                                                   |
 |  53  | [Maximum SubArray](https://leetcode.com/problems/maximum-subarray)                                                                                        |                                 [![Java](assets/java.png)](src/MaximumSubArray.java) [![Python](assets/python.png)](python/maximum_sum_subarray.py)                                  |                                                                                                                                   |
 |  55  | [Jump Game](https://leetcode.com/problems/jump-game)                                                                                                      |                                                                    [![Java](assets/java.png)](src/JumpGame.java)                                                                     |                                                                                                                                   |
 |  58  | [Length of Last Word](https://leetcode.com/problems/length-of-last-word)                                                                                  |                                 [![Java](assets/java.png)](src/LengthOfLastWord.java) [![Python](assets/python.png)](python/length_of_last_word.py)                                  |                                                                                                                                   |

src/Permutations.java

@@ -0,0 +1,76 @@
+// https://leetcode.com/problems/permutations
+// T: O(n * n!)
+// S: O(n * n!)
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class Permutations {
+    private static final Map<Integer, List<List<Integer>>> PERMUTATION_MASKS = new HashMap<>();
+
+    static {
+        PERMUTATION_MASKS.put(1, List.of(List.of(0)));
+        PERMUTATION_MASKS.put(2, List.of(
+                List.of(0, 1),
+                List.of(1, 0)
+        ));
+        PERMUTATION_MASKS.put(3, 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)
+        ));
+
+        for (int i = 4 ; i <= 6 ; i++) {
+            PERMUTATION_MASKS.put(i, getPermutationMask(i));
+        }
+    }
+
+    private static List<List<Integer>> getPermutationMask(int size) {
+        final List<List<Integer>> result = new ArrayList<>();
+        final List<List<Integer>> previousPermutations = PERMUTATION_MASKS.get(size - 1);
+        for (List<Integer> mask : previousPermutations) {
+            for (int insertionPosition = 0 ; insertionPosition < size ; insertionPosition++) {
+                result.add(newPermutation(mask, insertionPosition));
+            }
+        }
+        return result;
+    }
+
+    private static List<Integer> newPermutation(List<Integer> mask, int index) {
+        final List<Integer> result = new ArrayList<>();
+        for (int i = 0 ; i < index ; i++) result.add(mask.get(i));
+        result.add(mask.size());
+        for (int i = index ; i < mask.size() ; i++) result.add(mask.get(i));
+        return result;
+    }
+
+    public List<List<Integer>> permute(int[] nums) {
+        final List<List<Integer>> results = new ArrayList<>();
+        final List<List<Integer>> permutationMasks = PERMUTATION_MASKS.get(nums.length);
+        for (List<Integer> mask : permutationMasks) {
+            results.add(permute(nums, mask));
+        }
+        return results;
+    }
+
+    private List<Integer> permute(int[] array, List<Integer> mask) {
+        final int[] result = new int[array.length];
+        for (int i = 0 ; i < array.length ; i++) {
+            result[mask.get(i)] = array[i];
+        }
+        return toList(result);
+    }
+
+    private List<Integer> toList(int[] array) {
+        final List<Integer> result = new ArrayList<>(array.length);
+        for (int element : array) {
+            result.add(element);
+        }
+        return result;
+    }
+}