<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 solve(int idx, vector<int>&nums, vector<int>& temp, vector<vector<int>>& ans,unordered_set<int>& st, int k)
    {
        if(k == 0)
        {
            ans.push_back(temp);
            return;
        }
        if(idx == nums.size()) return;

        for(int i = idx+1; i<nums.size(); i++)
        {
            if(st.find(nums[i]) == st.end())
            {
                temp.push_back(nums[i]);
                st.insert(nums[i]);
                solve(i, nums, temp,ans, st, k-1);
                temp.pop_back();
                st.erase(nums[i]);
            }
        }
    }
    vector<vector<int>> combine(int n, int k) {
        vector<int> nums(n);
        vector<vector<int>> ans;
        unordered_set<int> st;
        vector<int> temp;
        for(int i = 0; i<n; i++) nums[i] = i + 1;
        for(int i = 0; i<n; i++)
        {
            temp.push_back(nums[i]);
            st.insert(nums[i]);
            solve(i, nums, temp, ans, st, k-1);
            temp.pop_back();
            st.erase(nums[i]);
        }

        return ans;

    }
};
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