39. Combination Sum

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

It is guaranteed that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

回溯，记录当前加和。
遍历所有数组元素，当sum大于target时返回，等于target时加入数组并返回。
每次遍历并回溯元素时只递归当前元素和其之后的元素。（防止重复。）

class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> ret = new ArrayList<>();
        backtracking(candidates, ret, new ArrayList<>(), target, 0, 0);
        return ret;
    }
    
    private void backtracking(int[] candidates, List<List<Integer>> ret, List<Integer> arr, int target, int sum, int level){
        if(sum > target ){
            return;
        }
        else if (sum == target){
            ret.add(new ArrayList<>(arr));
            return;
        }

        for(int i = level; i < candidates.length; i++){
            arr.add(candidates[i]);
            sum+= candidates[i];
            backtracking(candidates, ret, arr, target, sum, i);
            sum-= candidates[i];
            arr.remove(arr.size()-1);
        }
    }
}