solves combination sum

Author: anishLearnsToCode

Diff for: README.md

@@ -42,6 +42,7 @@
 |  35  | [Search Inserted Position](https://leetcode.com/problems/search-insert-position/)                                                                         |                              [![Java](assets/java.png)](src/SearchInsertPosition.java) [![Python](assets/python.png)](python/search_insert_position.py)                              |                                                                                                                                   |
 |  36  | [Valid Sudoku](https://leetcode.com/problems/valid-sudoku)                                                                                                |                                                                   [![Java](assets/java.png)](src/ValidSudoku.java)                                                                   |                                                                                                                                   |
 |  38  | [Count and Say](https://leetcode.com/problems/count-and-say)                                                                                              |                                       [![Java](assets/java.png)](src/CountAndSay.java) [![Python](assets/python.png)](python/count_and_say.py)                                       |                                                                                                                                   |
+|  39  | [Combination Sum](https://leetcode.com/problems/combination-sum)                                                                                          |                                                                 [![Java](assets/java.png)](src/CombinationSum.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)                                  |                                                                                                                                   |
 |  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)                                  |                                                                                                                                   |
 |  66  | [Plus One](https://leetcode.com/problems/plus-one)                                                                                                        |                                           [![Java](assets/java.png)](src/PlusOne.java) [![Python](assets/python.png)](python/plus_one.py)                                            |                                                                                                                                   |

Diff for: src/CombinationSum.java

@@ -0,0 +1,31 @@
+// https://leetcode.com/problems/combination-sum
+// T: O(|candidates| ^ target)
+// S: O(target + candidate ^ 2) // not sure tho --> if anyone has any idea please feel free to contact me
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class CombinationSum {
+    private final List<List<Integer>> result = new ArrayList<>();
+    private int[] candidates;
+    private int target;
+
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        this.candidates = candidates;
+        this.target = target;
+        combinationSum(new ArrayList<>(), 0, 0);
+        return result;
+    }
+
+    public void combinationSum(List<Integer> current, int sum, int index) {
+        if (sum == target) {
+            result.add(current);
+            return;
+        } else if (sum > target) return;
+        for (int i = index ; i < candidates.length ; i++) {
+            List<Integer> newCandidate = new ArrayList<>(current);
+            newCandidate.add(candidates[i]);
+            combinationSum(newCandidate, sum + candidates[i], i);
+        }
+    }
+}