class Solution {
    
    List<List<Integer>> result = new ArrayList<>();
    LinkedList<Integer> path = new LinkedList<>();
    public List<List<Integer>> combinationSum3(int k, int n) {
        
        backTracking(n,0,k,1);
        return result;

    }

    void backTracking(int n, int sum, int k, int startIndex) {
        if (path.size() == k) {
            if (sum == n) {
                result.add(new ArrayList<>(path));                
            }
        }

        if (sum > n) {
            return;
        }

        for (int i = startIndex; i <= 9; i++) {
            path.add(i);
            sum += i;
            backTracking(n, sum, k, i+1);
            sum -= i;
            path.removeLast();
        }

    }
}

class Solution {
    List<String> list = new ArrayList<>();
    
    public List<String> letterCombinations(String digits) {
        if (digits == null || digits.length() == 0) {
         return list;
     }   
        String[] numString = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        backTracking(digits, numString, 0);
        return list;

    }

    StringBuilder temp = new StringBuilder();

    void backTracking( String digits, String[] numString, int index) {
        if (index == digits.length()) {
            list.add(temp.toString());
            return;
        }
        String str = numString[(digits.charAt(index))-'0'];
        for (int i = 0; i < str.length(); i++) {
            temp.append(str.charAt(i));
            backTracking(digits, numString, index + 1);
            temp.deleteCharAt(temp.length() -1 );
        }

    }
}