Combination Sum II.java

/*
Given a collection of candidate numbers (candidates) and a target number (target), 
find all unique combinations in candidates where the candidate numbers sums to target.
Each number in candidates may only be used once in the combination.

Note:
All numbers (including target) will be positive integers.
The solution set must not contain duplicate combinations.

Example 1:
Input: candidates = [10,1,2,7,6,1,5], target = 8,
A solution set is:
[
  [1, 7],
  [1, 2, 5],
  [2, 6],
  [1, 1, 6]
]

Example 2:
Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
  [1,2,2],
  [5]
]

time = O(n^2)
space = O(n)
*/

class Solution {
    public List<List<Integer>> combinationSum2(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(candidates);
        dfs(result, new ArrayList<>(), 0, target, candidates);
        return result;
    }
    
    private void dfs(List<List<Integer>> result, List<Integer> list, int start, int remain, int[] candidates) {
        if (remain < 0) {
            return;
        }
        if (remain == 0) {
            result.add(new ArrayList<>(list));
            return;
        }
        for (int i = start; i < candidates.length; i++) {
            if (i == start || candidates[i] != candidates[i - 1]) {
                list.add(candidates[i]);
                dfs(result, list, i + 1, remain - candidates[i], candidates);
                list.remove(list.size() - 1);
            }
        }
    }
}