Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Note:
Example 1:
Input: k = 3, n = 7 Output: [[1,2,4]]
Example 2:
Input: k = 3, n = 9 Output: [[1,2,6], [1,3,5], [2,3,4]]
class Solution { public List<List<Integer>> combinationSum3(int k, int n) { List<List<Integer>> arrList = new ArrayList<>(); List<Integer> list = new ArrayList<>(); helper(arrList, list, 0, 1, k, n); return arrList; } private void helper(List<List<Integer>> arrList, List<Integer> list, int preSum, int index, int k, int n) { if (k == 0) { if (preSum == n) { arrList.add(new ArrayList<>(list)); } return; } for (int i = index; i < 10; i++) { list.add(i); helper(arrList, list, preSum + i, i + 1, k - 1, n); list.remove(list.size() - 1); } } }
原文：https://www.cnblogs.com/xuanlu/p/12320763.html