let 39 Combination Sum

Given a set of candidate numbers (C) (without duplicates) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:
All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.
For example, given candidate set [2, 3, 6, 7] and target 7,
A solution set is:
[
[7],
[2, 2, 3]
]

class Solution {
public List<List<Integer>> combinationSum(int[] candidates, int target) {
List<List<Integer>> ans=new ArrayList<>();

Arrays.sort(candidates);
backtrack(ans, new ArrayList<>(),  candidates, target, 0);
return ans;
}

public void backtrack(List<List<Integer>> ans,List<Integer> tmpList,int[] nums,int remain,int start){

if(remain<0) return ;
else if(remain==0) ans.add(new ArrayList<>(tmpList));
else{
for(int i=start;i<nums.length;i++){
tmpList.add(nums[i]);
backtrack(ans,tmpList,nums,remain-nums[i],i);// here start is important

tmpList.remove(tmpList.size()-1);
}
}
}
}