publicList<List<Integer>>combinationSum(int[]candidates,inttarget) {

List<List<Integer>>result =newArrayList();

Arrays.sort(candidates);

helper(candidates,target,0,newArrayList(),result);

backtracking(candidates,target,0,newArrayList(),result);

returnresult;

}

privatevoidhelper(int[]candidates,inttarget,intstartIndex,List<Integer>curr,List<List<Integer>>result){

privatevoidbacktracking(int[]candidates,inttarget,intstartIndex,List<Integer>curr,List<List<Integer>>result){

if(target >0){

intprev = -1;

for(inti =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);

}

}elseif(target ==0){

|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