题目描述

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]]

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

分析

参考：LeetCode 039 Combination Sum、LeetCode 040 Combination Sum II

代码

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

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

        if (k < 1 || n < 1) {
            return rt;
        }

        List<Integer> cur = new ArrayList<Integer>();
        dfs(rt, cur, 0, k, n, 1);
        return rt;
    }

    private static void dfs(List<List<Integer>> rt, List<Integer> cur, int sum,
            int k, int n, int level) {

        if (sum == n && k == 0) {
            rt.add(new ArrayList<Integer>(cur));
            return;
        } else if (sum > n || k <= 0) {
            return;
        }

        for (int i = level; i <= 9; i++) {
            cur.add(i);
            dfs(rt, cur, sum + i, k - 1, n, i + 1);
            cur.remove(cur.size() - 1);
        }

    }