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+<h2><a href="https://leetcode.com/problems/combination-sum/">39. Combination Sum</a></h2><h3>Medium</h3><hr><div><p>Given an array of <strong>distinct</strong> integers <code>candidates</code> and a target integer <code>target</code>, return <em>a list of all <strong>unique combinations</strong> of </em><code>candidates</code><em> where the chosen numbers sum to </em><code>target</code><em>.</em> You may return the combinations in <strong>any order</strong>.</p>
+
+<p>The <strong>same</strong> number may be chosen from <code>candidates</code> an <strong>unlimited number of times</strong>. Two combinations are unique if the frequency of at least one of the chosen numbers is different.</p>
+
+<p>The test cases are generated such that the number of unique combinations that sum up to <code>target</code> is less than <code>150</code> combinations for the given input.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre><strong>Input:</strong> candidates = [2,3,6,7], target = 7
+<strong>Output:</strong> [[2,2,3],[7]]
+<strong>Explanation:</strong>
+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.
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre><strong>Input:</strong> candidates = [2,3,5], target = 8
+<strong>Output:</strong> [[2,2,2,2],[2,3,3],[3,5]]
+</pre>
+
+<p><strong>Example 3:</strong></p>
+
+<pre><strong>Input:</strong> candidates = [2], target = 1
+<strong>Output:</strong> []
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= candidates.length &lt;= 30</code></li>
+	<li><code>2 &lt;= candidates[i] &lt;= 40</code></li>
+	<li>All elements of <code>candidates</code> are <strong>distinct</strong>.</li>
+	<li><code>1 &lt;= target &lt;= 40</code></li>
+</ul>
+</div>