216. Combination Sum III (Medium)

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.

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]]

Hints

Solutions

  • Java
class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {
        if (k <= 0) {
            return new LinkedList<>();
        }

        List<List<Integer>> rst = new LinkedList<>();
        dfs(1, n, k, new LinkedList<>(), rst);
        return rst;
    }

    private void dfs(int start, int remainTarget, int k, LinkedList<Integer> list, List<List<Integer>> rst) {
        if (list.size() == k && remainTarget == 0) {
            rst.add(new LinkedList<>(list));
            return;
        }

        for (int i = start; i <= 9; i++) {
            if (i > remainTarget) {
                break;
            }
            list.add(i);
            dfs(i + 1,remainTarget - i, k, list, rst);
            list.remove(list.size() - 1);
        }
    }
}
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