Combination Sum III(Medium)

LeetCode Source

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]]

解题思路：回溯+DFS

public List<List<Integer>> combinationSum3(int k, int n) {
    List<List<Integer>> res = new ArrayList<List<Integer>>();
    if (n <= 0 || k <= 0) {
        return res;
    }
    if (k > 9 || n > 55 )
        return res;

    List<Integer> cur = new ArrayList<Integer>();
    helper(cur, res, 1, n, k);
    return res;
}


public void helper(List<Integer> current, List<List<Integer>> res, int index, int target, int k) {
    if (current.size() == k && target == 0) {
        res.add(new ArrayList<Integer>(current));
        return;
    }

    if (current.size() > k || target < 0) {
        return;
    }

    for (int i = index; i <= 9; i++) {
        current.add(i);
        helper(current, res, i + 1, target - i, k);
        current.remove(current.size() - 1);
    }
}

References

1 Combinations