216. Combination Sum III

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.

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]]

本题在combination2的基础上，添加了元素个数，比较简单，代码如下：

public class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        int[] num = new int[]{1,2,3,4,5,6,7,8,9};
        backtracking(res,new ArrayList<Integer>(),num,k,n,0);
        return res;
    }
    public void backtracking(List<List<Integer>> res,List<Integer> list,int[] num,int k,int target,int start){
        if(list.size()>k) return;
        if(target<0) return;
        else if(target==0){
            if(list.size()==k)
            res.add(new ArrayList<Integer>(list));
        }else{
            for(int i=start;i<num.length;i++){
                list.add(num[i]);
                backtracking(res,list,num,k,target-num[i],i+1);
                list.remove(list.size()-1);
            }
        }
    }
}