<p>Given two integers <code>n</code> and <code>k</code>, return <em>all possible combinations of</em> <code>k</code> <em>numbers chosen from the range</em> <code>[1, n]</code>.</p>

<p>You may return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 4, k = 2
<strong>Output:</strong> [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
<strong>Explanation:</strong> 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.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1, k = 1
<strong>Output:</strong> [[1]]
<strong>Explanation:</strong> There is 1 choose 1 = 1 total combination.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 20</code></li>
	<li><code>1 &lt;= k &lt;= n</code></li>
</ul>

class Solution {
public:
        vector<vector<int>> res;

void help(vector<int> a,int idx,vector<int> &ans,int k){
        if(ans.size() == k){
            res.push_back(ans);
            return;
        }
        for(int i  = idx ;i<a.size();i++){
            ans.push_back(a[i]);
            help(a,i+1,ans,k);
            ans.pop_back();
        }

    }

    vector<vector<int>> combine(int n, int k) {
        vector<int> ans,a;
        for(int i=0;i<n;i++)
            a.push_back(i+1);
        help(a,0,ans,k);
        return res;
    }
};
166387