solves combination iii in java

Author: anishLearnsToCode

README.md

@@ -174,7 +174,7 @@
 | 211  | [Design Add and Search Words Data Structure](https://leetcode.com/problems/design-add-and-search-words-data-structure)                                    |                                                      [![Java](assets/java.png)](src/DesignAddAndSearchWordsDataStructure.java)                                                       |                                                                                                                                   |
 | 213  | [House Robber II](https://leetcode.com/problems/house-robber-ii)                                                                                          |                                                                  [![Java](assets/java.png)](src/HouseRobberII.java)                                                                  |                                                                                                                                   |
 | 215  | [Kth Largest Element in an Array](https://leetcode.com/problems/kth-largest-element-in-an-array)                                                          |                                                           [![Java](assets/java.png)](src/KthLargestElementInAnArray.java)                                                            |                                                                                                                                   |
-| 216  | [Combination Sum III](https://leetcode.com/problems/combination-sum-iii)                                                                                  |                                                                                                                                                                                      |                                                                                                                                   |
+| 216  | [Combination Sum III](https://leetcode.com/problems/combination-sum-iii)                                                                                  |                                                                [![Java](assets/java.png)](src/CombinationSumIII.java)                                                                |                                                                                                                                   |
 | 217  | [Contains Duplicate](https://leetcode.com/problems/contains-duplicate)                                                                                    |                                 [![Java](assets/java.png)](src/ContainsDuplicate.java) [![Python](assets/python.png)](python/contains_duplicate.py)                                  |                                                                                                                                   |
 | 219  | [Contains Duplicate II](https://leetcode.com/problems/contains-duplicate-ii)                                                                              |                               [![Java](assets/java.png)](src/ContainsDuplicateII.java) [![Python](assets/python.png)](python/contains_duplicate_ii.py)                               |                                                                                                                                   |
 | 220  | [Contains Duplicate III](https://leetcode.com/problems/contains-duplicate-iii)                                                                            |                                                                                                                                                                                      |                                                                                                                                   |

src/CombinationSumIII.java

@@ -0,0 +1,37 @@
+// https://leetcode.com/problems/combination-sum-iii
+// T: O(9^k)
+// S: O(k)
+
+import java.util.ArrayList;
+import java.util.LinkedList;
+import java.util.List;
+
+public class CombinationSumIII {
+    private final List<List<Integer>> result = new ArrayList<>();
+    private int sum;
+
+    public List<List<Integer>> combinationSum3(int k, int n) {
+        sum = n;
+        combinationSum(1, Math.min(sum / 2, 9), 0, k - 1, new LinkedList<>());
+        return result;
+    }
+
+    private void combinationSum(int start, int end, int currentSum, int k, LinkedList<Integer> current) {
+        if (k == 0) {
+            if (currentSum < sum && sum - currentSum >= start && sum - currentSum <= 9) {
+                current.add(sum - currentSum);
+                result.add(new ArrayList<>(current));
+                current.pollLast();
+            }
+            return;
+        }
+
+        for (int i = start ; i <= end ; i++) {
+            current.add(i);
+            currentSum += i;
+            combinationSum(i + 1, Math.min((sum - currentSum) / 2, 9), currentSum, k - 1, current);
+            currentSum -= i;
+            current.pollLast();
+        }
+    }
+}