Combinations

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
You may return the answer in any order.

Example 1:
Input: n = 4, k = 2
Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
Explanation: 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.

Example 2:
Input: n = 1, k = 1
Output: [[1]]
Explanation: There is 1 choose 1 = 1 total combination.
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// tc=O(C(n,k)*k) sc=O(k) auxiliary
class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<int>res;
        vector<vector<int>>ans;
        solve(1, n, k, res, ans);
        return ans;
    }

    void solve(int idx, int n, int k, vector<int>&res, vector<vector<int>>& ans){
        if(res.size()==k){
            ans.push_back(res);
            return; // if you dont use this  then the tc=O(2^n) sc=O(k) if we dont use return then ou continue recursion even after res.size() == k.
        }

        for(int i=idx;i<=n;i++){
            res.push_back(i);
            solve(i+1, n,k,res,ans);
            res.pop_back();
        }
    }
};