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:All numbers will be positive integers.The solution set must not contain duplicate combinations.Example 1:Input: k = 3, n = 7Output: [[1,2,4]]Example 2:Input: k = 3, n = 9Output: [[1,2,6], [1,3,5], [2,3,4]]难度：medium题目：找出所有可能的Ｋ个数之和为n的组合，只可以使用1到9之间的数。注意：所有数都为正，给出的答案中不包含重复的数。思路：递归Runtime: 1 ms, faster than 66.65% of Java online submissions for Combination Sum III.Memory Usage: 35 MB, less than 54.02% of Java online submissions for Combination Sum III.class Solution { public List<List<Integer>> combinationSum3(int k, int n) { List<List<Integer>> result = new ArrayList<>(); if (n > 45 || n < 1 || k < 1 || k > 9) { return result; } combination(1, k, 0, n, new Stack<>(), result); return result; } private void combination(int begin, int k, int sum, int n, Stack<Integer> stack, List<List<Integer>> result) { if (k < 0 || sum > n) { return; } if (0 == k && sum == n) { result.add(new ArrayList<>(stack)); return; } for (int i = begin; i <= 9 - k + 1; i++) { stack.push(i); combination(i + 1, k - 1, sum + i, n, stack, result); stack.pop(); } }}