39. Combination Sum

Given a set of candidate numbers (C) 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]
]

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        List<Integer> member = new ArrayList<>();
        if(candidates.length == 0)
            return res;
        dfs(res, member, candidates, target, 0);
        return res;
    }
    public void dfs(List<List<Integer>> res, List<Integer> member, int[] candidates, int target, int deep){
        if(target < 0)
            return;
        if(target == 0){
            res.add(new ArrayList<Integer>(member));
            return;
        }
        for(int i = deep; i < candidates.length; i++){
            member.add(candidates[i]);
            dfs(res, member, candidates, target-candidates[i], i);
            member.remove(member.size()-1);
        }
    }
}