Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:
[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ]
class Solution {
public:
vector<vector<int> > result;
vector<int> row;
void dfs_combine(const int &n, const int &k, int first) {
if (row.size() == k) {
result.push_back(row);
return;
}
int i;
for (i=first;i<=n;i++) {
row.push_back(i);
dfs_combine(n, k, i+1);
row.pop_back();
}
}
vector<vector<int> > combine(int n, int k) {
result.clear();
row.clear();
dfs_combine(n,k,1);
return result;
}
};