LeetCode-Combination Sum III

这个题和上个题以及 combination那个题几乎一样 只需要注意细节 只能有1-9 并且弄清楚 n 和 k含义

public class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<List<Integer>> ();
        List<Integer> list = new ArrayList<Integer>();
        helper ( res, list, 1, k, n);
        return res;
        
    }
    public void helper ( List<List<Integer>> res, List<Integer> list, int next, int k, int n ){
        if ( n == 0 && list.size() == k ) {
            res.add ( new ArrayList<Integer> (list));
            return;
        }
        while ( next <= 9 && n >= next ){
            list.add ( next );
            helper ( res, list, next + 1, k, n - next);
            list.remove( list.size() - 1 );
            next ++;
        }
        
    }
}