39. Combination Sum

class Solution {
    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);
        List<Integer> cur = new ArrayList<>();
        helper(candidates, target, 0, cur, res);
        return res;
    }
    
    private void helper(int[] candidates, int target, int index, List<Integer> cur, List<List<Integer>> res) {
        if (getSum(cur) > target) {
            return;
        }
        if (getSum(cur) == target) { 
            res.add(new ArrayList<>(cur));
            return;
        }
        
        // 从 index 开始取，保证取到的下一位不小于当前位，避免结果出现 [2,2,3] 和 [2,3,2] 的重复
        for (int i = index; i < candidates.length; i++) {
            cur.add(candidates[i]);
            // 可以取重复元素，所以这里传 i，而不是 i + 1
            helper(candidates, target, i, cur, res); 
            cur.remove(cur.size() - 1);
        }
    }
    
    private int getSum(List<Integer> list) {
        int sum = 0;
        for (int i = 0; i < list.size(); i++) {
            sum += list.get(i);
        }
        return sum;
    }
}