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:
代码语言：txt复制[1,2,6, 1,3,5, 2,3,4]
这次是指定了元素的个数，用一个变量记录即可。
public List<List<Integer>> combinationSum3(int k, int n) {
List<List<Integer>> rt = new ArrayList<List<Integer>>();
if (k < 1 || n < 1) {
return rt;
}
List<Integer> cur = new ArrayList<Integer>();
dfs(rt, cur, 0, k, n, 1);
return rt;
}
private void dfs(List<List<Integer>> rt, List<Integer> cur, int sum, int k,
int n, int level) {
if (sum == n && k == 0) {
rt.add(new ArrayList<Integer>(cur));
return;
} else if (sum > n || k <= 0) {
return;
}
for (int i = level; i <= 9; i ) {
cur.add(i);
dfs(rt, cur, sum i, k - 1, n, i 1);
cur.remove(cur.size() - 1);
}
}
