Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbersfrom 1 to 9 can be used and each combination should be a unique set of numbers.Ensure that numbers within the set are sorted in ascending order.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]]

public class Solution {public List<List<Integer>> combinationSum3(int k, int n) {List<List<Integer>> res=new ArrayList<List<Integer>>();if(n>45 || k>9) return res;List<Integer> temp=new ArrayList<Integer>();select_num(k,1,n, temp, res);return res;}public void select_num(int k,int idx,int n, List<Integer> temp, List<List<Integer>> res){if(n<0) return;if(k==0&& n==0){List<Integer> temp_new=new ArrayList<Integer>(temp);res.add(temp_new);return;}if(idx>9) return;temp.add(idx);select_num(k-1,idx+1,n-idx,temp,res);temp.remove(temp.size()-1);select_num(k,idx+1,n,temp,res);}}

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