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]]
这是一个多阶段问题，目标是求所有解，显然用深搜+剪枝，即回溯法。
{% if book.java %} {% codesnippet "./code/combination-sum-iii."+book.suffix, language=book.suffix %}{% endcodesnippet %} {% endif %}