leetcode 77 - Combinations

https://leetcode.com/problems/combinations/

Given two integers n and k, return all possible combinations of k numbers out of 1 … n.

You may return the answer in any order.

Example 1:

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

Input: n = 1, k = 1
Output: [[1]]

Constraints:

1 <= n <= 20
1 <= k <= n

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

vector<vector<int>> combine(int n, int k) {
vector<int> result;

combine(n, k, 1, result);
return all_result;
}

void combine(int n, int k, int i, vector<int> &result) {
if (result.size() == k) {
all_result.push_back(result);
return;
}

if (i > n)
return;

for (int j = i; j <= n; j++) {
result.push_back(j);
combine(n, k, j + 1, result);
result.pop_back();
}
}
};