Combination Sum.java

/*
Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), 
find all unique combinations in candidates where the candidate numbers sums to target.
The same repeated number may be chosen from candidates unlimited number of times.

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

Example 1:
Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]

Example 2:
Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]


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

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (candidates == null) {
            return result;
        }
        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++) {
            list.add(candidates[i]);
            dfs(result, list, i, remain - candidates[i], candidates);
            list.remove(list.size() - 1);
        }
    }
}