LC.39.Combination Sum

https://leetcode.com/problems/combination-sum/description/

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]
]

time: o(2^n)
space: o(n)

public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        dfs(candidates, target, 0, res, new ArrayList<>());
        return res ;
    }
    private void dfs(int[] candidates, int remain,int start, List<List<Integer>> res, List<Integer> sol){
        //terminate condition
        if (remain <= 0) {
            if (remain == 0) {
                res.add(new ArrayList(sol));
            }
            return;
        }
        for (int i = start ; i < candidates.length; i++) {
               sol.add(candidates[i]);
               //因为允许重复，所以用i
               dfs(candidates, remain-candidates[i], i, res, sol);
               sol.remove(sol.size()-1);
        }
    }