Combinations
Difficulty: Medium
Category: DSA
Topics: Backtracking
Asked at: Facebook, Amazon
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.
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**Example 2:**
**Input:** n = 1, k = 1
**Output:** [[1]]
**Explanation:** There is 1 choose 1 = 1 total combination.
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**Constraints:**
- `1 <= n <= 20`
- `1 <= k <= n`