<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:
    void helper(vector<vector<int>>& ans, int n, int k, vector<int>& curr, vector<bool>& flag , int prev ) {
        if (k == 0) {
            vector<int> nC = curr;
            ans.push_back(nC);
            return;
        }


        for(int i =  prev + 1; i <= n ; i ++){
            if(flag[i]  ){
                curr.push_back(i);
                flag[i] = false;
                helper(ans, n, k-1, curr, flag, i );
                flag[i] = true;
                curr.pop_back();
            }
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> ans;
        vector<int> curr;
        vector<bool> flag (n +1, true);
        helper(ans, n, k , curr, flag , 0);
        return ans;
    }
};
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