题解 | #加起来和为目标值的组合#

import java.util.*;


public class Solution {

    private ArrayList<ArrayList<Integer>> res = new ArrayList<>();

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param target int整型 
     * @param nums int整型一维数组 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> combinationCount (int target, int[] nums) {
        // write code here
        if (nums == null || nums.length < 1) {
            return res;
        }
        Arrays.sort(nums);
        backtrack(nums, 0, target, new ArrayList<>());
        return res;
    }

    private void backtrack(int[] nums, int start, int target, List<Integer> list) {
        if (list.size() > 0 && target == 0) {
            res.add(new ArrayList<>(list));
            return;
        }
        for (int i = start; i < nums.length; i++) {
            if (target - nums[i] < 0) {
                break;
            }
            target -= nums[i];
            list.add(nums[i]);
            backtrack(nums, i, target, list);
            target += nums[i];
            list.remove(list.size() - 1);
        }
    }
}