40. Combination Sum II

class Solution {
    public List<List<Integer>> combinationSum2(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;
        }
        for (int i = index; i < candidates.length; i++) {
            //  很重要的去重
            if (i != index && candidates[i] == candidates[i - 1]) {
                continue;
            }
            cur.add(candidates[i]);
            helper(candidates, target, i + 1, cur, res); // 这里传 i + 1，从 i + 1 开始取，避免一个值取多次
            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;
    }
}