combination sum

fishercoder1534 · fishercoder1534 · commit ab8aefcc0741 · 2016-12-03T17:33:38.000-08:00
diff --git a/MEDIUM/src/medium/CombinationSum.java b/MEDIUM/src/medium/CombinationSum.java
@@ -9,15 +9,18 @@ public class CombinationSum {
     public List<List<Integer>> combinationSum(int[] candidates, int target) {
         List<List<Integer>> result = new ArrayList();
         Arrays.sort(candidates);
-        helper(candidates, target, 0, new ArrayList(), result);
+        backtracking(candidates, target, 0, new ArrayList(), result);
         return result;
     }
     
-    private void helper(int[] candidates, int target, int startIndex, List<Integer> curr, List<List<Integer>> result){
+    private void backtracking(int[] candidates, int target, int startIndex, List<Integer> curr, List<List<Integer>> result){
         if(target > 0){
+            int prev = -1;
             for(int i = startIndex; i < candidates.length; i++){
+                if (candidates[i] > target) return;//this is one very important step to optimize this algorithm: pruning
+                if (prev != -1 && prev == candidates[i]) continue;
                 curr.add(candidates[i]);
-                helper(candidates, target-candidates[i], i, curr, result);
+                backtracking(candidates, target-candidates[i], i, curr, result);
                 curr.remove(curr.size()-1);
             }
         } else if(target == 0){
diff --git a/README.md b/README.md
@@ -119,6 +119,7 @@
 |56|[Merge Intervals](https://leetcode.com/problems/merge-intervals/)|[Solution](../../blob/master/HARD/src/hard/MergeIntervals.java)|O(n*logn)|O(1)|Hard|
 |47|[Permutations II](https://leetcode.com/problems/permutations-ii/)|[Solution](../../blob/master/MEDIUM/src/medium/PermutationsII.java)|O(n*n!)|O(n)|Medium|Backtracking
 |43|[Multiply Strings](https://leetcode.com/problems/multiply-strings/)|[Solution]|||Medium
+|39|[Combination Sum](https://leetcode.com/problems/combination-sum/)|[Solution](../../blob/master/MEDIUM/src/medium/CombinationSum.java)|O(k*n^k)|O(k)|Medium|Backtracking
 |31|[Next Permutation](https://leetcode.com/problems/next-permutation)|[Solution](../../blob/master/MEDIUM/src/medium/NextPermutation.java)|O(n)|O(1)|Medium|Array
 |24|[Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/)|[Solution](../../blob/master/EASY/src/easy/SwapNodesinPairs.java)|O(n)|O(1)|Easy| Recursion, LinkedList
 |23|[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)|[Solution](../../blob/master/HARD/src/hard/MergeKSortedList.java)|O(n*logk)|O(logk)|Hard|Heap
