216. Combination Sum III

Problem:

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 = 7
Output: [[1,2,4]]

Example 2:

Input: k = 3, n = 9
Output: [[1,2,6], [1,3,5], [2,3,4]]

Analysis:
Combination sum 马甲题，顺便复习了一下DFS.

Solution:

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<>();
        dfs(res, new ArrayList<>(), 1, n, k);
        return res;
    }

    private void dfs(List<List<Integer>> res,  List<Integer> list,int index, int target, int k) {
        if (target == 0 && list.size() == k) {
            res.add(new ArrayList<>(list));
            return;
        } 
        if (target < 0 || list.size() > k) return;
        for (int i = index; i <= 9; i++) {
            list.add(i);
            dfs(res, list,i + 1, target - i, k);
            list.remove(list.size() - 1);
        }
    }
}