solves combinations

Author: anishLearnsToCode

README.md

@@ -70,6 +70,7 @@
 |  73  | [Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes)                                                                                      |                                                                 [![Java](assets/java.png)](src/SetMatrixZeroes.java)                                                                 |                                                                                                                                   |
 |  74  | [Search a 2D Matrix](https://leetcode.com/problems/search-a-2d-matrix)                                                                                    |                                                                 [![Java](assets/java.png)](src/SearchA2DMatrix.java)                                                                 |                                                                                                                                   |
 |  75  | [Sort Colors](https://leetcode.com/problems/sort-colors)                                                                                                  |                                                                   [![Java](assets/java.png)](src/SortColors.java)                                                                    |                                                                                                                                   |
+|  77  | [Combinations](https://leetcode.com/problems/combinations)                                                                                                |                                                                  [![Java](assets/java.png)](src/Combinations.java)                                                                   |                                                                                                                                   |
 |  83  | [Remove Duplicates from Sorted List](https://leetcode.com/problems/remove-duplicates-from-sorted-list)                                                    |                   [![Java](assets/java.png)](src/RemoveDuplicatesFromSortedList.java) [![Python](assets/python.png)](python/remove_duplicates_from_linked_list.py)                   |                                                                                                                                   |
 |  88  | [Merge Sorted Array](https://leetcode.com/problems/merge-sorted-array)                                                                                    |                                  [![Java](assets/java.png)](src/MergeSortedArray.java) [![Python](assets/python.png)](python/merge_sorted_array.py)                                  |                                                                                                                                   |
 |  94  | [Binary Tree Inorder Traversal](https://leetcode.com/problems/binary-tree-inorder-traversal/)                                                             |                       [![Java](assets/java.png)](src/BinaryTreeInorderTraversal.java) [![Python](assets/python.png)](python/binary_tree_inorder_traversal.py)                        |                                                                                                                                   |

src/Combinations.java

@@ -0,0 +1,31 @@
+// https://leetcode.com/problems/combinations
+// T: O(k * 2^n)
+// S: O(k)
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+public class Combinations {
+    public List<List<Integer>> combine(int n, int k) {
+        final List<List<Integer>> result = new ArrayList<>();
+        createCombinations(n, k, result);
+        return result;
+    }
+
+    private void createCombinations(int n, int k, List<List<Integer>> result) {
+        addCombinations(n, 1, k, result, new LinkedList<>());
+    }
+
+    private void addCombinations(int n, int current, int length, List<List<Integer>> result, LinkedList<Integer> combination) {
+        if (combination.size() == length) {
+            result.add(new ArrayList<>(combination));
+            return;
+        }
+        if (current > n) return;
+        combination.add(current);
+        addCombinations(n, current + 1, length, result, combination);
+        combination.removeLast();
+        addCombinations(n, current + 1, length, result, combination);
+    }
+}