Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
Example:
Input: n = 4, k = 2 Output: [ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
AC code:
class Solution {
public:
vector<vector<int>> combine(int n, int k) {
vector<vector<int>> res;
vector<int> temp;
solvecombine(1, n, k, res, temp);
return res;
}
void solvecombine(int begin, int n, int k, vector<vector<int>>& res, vector<int>& temp) {
if (k == 0) {
res.push_back(temp);
return;
}
for (int i = begin; n-i+1 >= k; ++i) {
temp.push_back(i);
solvecombine(i+1, n, k-1, res, temp);
temp.pop_back();
}
}
};
Runtime: 88 ms, faster than 42.24% of C++ online submissions for Combinations.