39\. Combination Sum

Medium

Given an array of **distinct** integers `candidates` and a target integer `target`, return _a list of all **unique combinations** of_ `candidates` _where the chosen numbers sum to_ `target`_._ You may return the combinations in **any order**.

The **same** number may be chosen from `candidates` an **unlimited number of times**. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

It is **guaranteed** that the number of unique combinations that sum up to `target` is less than `150` combinations for the given input.

**Example 1:**

**Input:** candidates = [2,3,6,7], target = 7

**Output:** [[2,2,3],[7]]

**Explanation:**

    2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
    7 is a candidate, and 7 = 7.
    These are the only two combinations. 

**Example 2:**

**Input:** candidates = [2,3,5], target = 8

**Output:** [[2,2,2,2],[2,3,3],[3,5]] 

**Example 3:**

**Input:** candidates = [2], target = 1

**Output:** [] 

**Example 4:**

**Input:** candidates = [1], target = 1

**Output:** [[1]] 

**Example 5:**

**Input:** candidates = [1], target = 2

**Output:** [[1,1]] 

**Constraints:**

*   `1 <= candidates.length <= 30`
*   `1 <= candidates[i] <= 200`
*   All elements of `candidates` are **distinct**.
*   `1 <= target <= 500`