第77題，題目如下:
Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
You may return the answer in any order.
Example 1:
Input: n = 4, k = 2 Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]] Explanation: There are 4 choose 2 = 6 total combinations. Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
Example 2:
Input: n = 1, k = 1 Output: [[1]] Explanation: There is 1 choose 1 = 1 total combination.
Constraints:
1 <= n <= 201 <= k <= n
搜尋，搜到尾的那種，每次搜尋查找一定要取到k個，讓i=s…n去運作
public List<List<Integer>> combine(int n, int k) {
List<List<Integer>> ans = new ArrayList<>();
dfs(n, k, 1, new ArrayList<>(), ans);
return ans;
}
private void dfs(int n, int k, int s, List<Integer> path, List<List<Integer>> ans) {
if (path.size() == k) {
ans.add(new ArrayList<>(path));
return;
}
for (int i = s; i <= n; ++i) {
path.add(i);
dfs(n, k, i + 1, path, ans);
path.remove(path.size() – 1);
}
}
