39. Combination Sum

Given a set of candidate numbers (C)(without duplicates)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 target7,
A solution set is:

[
  [7],
  [2, 2, 3]
]

思路： 递归

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> listResult = new ArrayList<List<Integer>>(); 
        List<Integer> temp = new ArrayList<Integer>();
        Arrays.sort(candidates);
        backtrack(listResult,temp,candidates,0,0,target);
        return listResult;
    }

    public void backtrack(List<List<Integer>> listResult,List<Integer> temp, int[] arr, int sum, int start, int target){
        if (sum==target){
            listResult.add(new ArrayList<>(temp));
        }else if(sum>target){
            return;
        }else{
            for (int i=start;i<arr.length;i++){
                temp.add(arr[i]);
                backtrack(listResult,temp,arr,sum + arr[i],i,target);
                temp.remove(temp.size()-1);
            }
        }
    }
}