Combination Sum III

https://leetcode.com/problems/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.

Ensure that numbers within the set are sorted in ascending order.

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]]

解题思路：

新题，与前面的系列相比，简单了一些，常规的dfs，没有啥好说的。

public class Solution {

    public List<List<Integer>> combinationSum3(int k, int n) {

        List<List<Integer>> res = new ArrayList<List<Integer>>();

        List<Integer> cur = new ArrayList<Integer>();

        dfs(k, n, res, cur, 1, 0);

        return res;

    }

    

    public void dfs(int k, int n, List<List<Integer>> res, List<Integer> cur, int step, int sum) {

        if(cur.size() == k && sum == n) {

            res.add(new ArrayList(cur));

            return;

        }

        if(cur.size() > k) {

            return;

        }

        if(cur.size() <= k && sum > n) {

            return;

        }

        for(int i = step; i < 10; i++) {

            cur.add(i);

            sum += i;

            dfs(k, n, res, cur, i + 1, sum);

            cur.remove(cur.size() - 1);

            sum -= i;

        }

    }

}