Given a set of candidate numbers (C) 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 target 7,
A solution set is:
[
[7],
[2, 2, 3]
]

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public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        Arrays.sort(candidates);
        List<List<Integer>> result = new ArrayList<>();
        sum(result, new ArrayList<Integer>(), candidates, target, 0);
        return result;
    }
    
    public void sum(List<List<Integer>> result, ArrayList<Integer> cur, int[] candidates, int target, int start) {
        if (target == 0) {
            result.add(new ArrayList<Integer>(cur)); // same object in all recurisve calls
        } else if (target > 0) {
            for (int i = start; i < candidates.length; i++) {
                cur.add(candidates[i]);
                sum(result, cur, candidates, target - candidates[i], i);
                cur.remove(cur.size() - 1);
            }
        }
    } 
}