39. Combination Sum (Medium) Uber Snapchat

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 target7,
A solution set is:

[
  [7],
  [2, 2, 3]
]

Solution 1: DFS O(n * 2^n); O(n)

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<>();
        if (candidates == null || candidates.length == 0) {
            return res;
        }

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

    private void dfs(List<List<Integer>> res, List<Integer> comb, int[] candidates, int start, int remain) {
//        if (remain < 0) {
//            return;
//        }
        if (remain == 0) {
            res.add(new ArrayList<>(comb));
        }

        for (int i = start; i < candidates.length; i++) {
            if (candidates[i] > remain) {
                break;
            }
            comb.add(candidates[i]);
            dfs(res, comb, candidates, i, remain - candidates[i]); //数组无重复，每个数可用多次
            comb.remove(comb.size() - 1);
        }
    }