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]]
题目大意：从1~9找出所有的k个数的组合，使其和等于n。
解题思路：还是回溯，每个数只能出现一次，每次递归都从当前数的下一个开始，当k==0&&n==0，就可以加入结果集。
public List<List<Integer>> combinationSum3(int k, int n) {
List<List<Integer>> res = new ArrayList<>();
Deque<Integer> tmp = new ArrayDeque<>();
if (n == 0 || k == 0 || n / k > 9) {
return res;
}
helper(res, tmp, 1, k, n);
return res;
}
private void helper(List<List<Integer>> res, Deque<Integer> tmp, int start, int k, int n) {
if (0 == n && k == 0) {
res.add(new ArrayList<>(tmp));
// System.out.println(tmp);
return;
}
for (int i = start; i <= 9; i++) {
tmp.addLast(i);
helper(res, tmp, i + 1, k - 1, n - i);
tmp.removeLast();
}
}