solves combination sum ii

Author: anishLearnsToCode

Diff for: README.md

@@ -43,6 +43,7 @@
 |  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)                                                                  |                                                                                                                                   |
+|  40  | [Combination Sum II](https://leetcode.com/problems/combination-sum-ii)                                                                                    |                                                                [![Java](assets/java.png)](src/CombinationSumII.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/CombinationSumII.java

@@ -0,0 +1,39 @@
+// https://leetcode.com/problems/combination-sum-ii
+// T: O(2^N)
+// S: O(N)
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.List;
+
+public class CombinationSumII {
+    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
+        List<List<Integer>> results = new ArrayList<>();
+        LinkedList<Integer> comb = new LinkedList<>();
+        Arrays.sort(candidates);
+        backtrack(candidates, comb, target, 0, results);
+        return results;
+    }
+
+    private void backtrack(int[] candidates, LinkedList<Integer> comb,
+                           int remain, int curr,
+                           List<List<Integer>> results) {
+        if (remain == 0) {
+            results.add(new ArrayList<Integer>(comb));
+            return;
+        }
+
+        for (int nextCurr = curr; nextCurr < candidates.length; ++nextCurr) {
+            if (nextCurr > curr && candidates[nextCurr] == candidates[nextCurr - 1])
+                continue;
+
+            int pick = candidates[nextCurr];
+            if (remain - pick < 0) break;
+
+            comb.addLast(pick);
+            backtrack(candidates, comb, remain - pick, nextCurr + 1, results);
+            comb.removeLast();
+        }
+    }
+}