[LeetCode] 216. 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.

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]]

题意 从1-9中选取 k 个数字 使得这k个数字的和等于n
做法最先想到的就是回溯法了，因为是1-9 不重复的选取，可以看作1-9已经是排好序的，递归回溯判断即可

Java代码

class Solution {

    public List<List<Integer>> combinationSum3(int k, int n) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        List<Integer> cur = new ArrayList<>();
        sovle(res, cur, 0, k, n, 0);
        return res;
    }

    private void sovle(List<List<Integer>> res, List<Integer> cur, int sum, int k, int n, int lev) {
        if (sum == n && k == 0) {
            res.add(cur);
            return;
        }
        if (sum > n || k < 0)
            return;
        for (int i = lev; i <= 9; i++) {
            cur.add(i);
            sovle(res, cur, sum+i, k-1, n, i + 1);
            cur.remove(cur.size() - 1);
        }
    }
}