给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。
示例:
输入: n = 4, k = 2
输出:
[
[2,4],
[3,4],
[2,3],
[1,2],
[1,3],
[1,4],
]
代码
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> res;
vector<int> out;
helper(n, k, 1, out, res);
return res;
}
void helper(int n, int k, int level, vector<int>& out, vector<vector<int>>& res) {
if (out.size() == k) res.push_back(out);
for (int i = level; i <= n; ++i) {
out.push_back(i);
helper(n, k, i + 1, out, res);
out.pop_back();
}
}
};
