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leetcode/problem/combination-sum-iv.html

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+<p>Given an array of <strong>distinct</strong> integers <code>nums</code> and a target integer <code>target</code>, return <em>the number of possible combinations that add up to</em>&nbsp;<code>target</code>.</p>
+
+<p>The test cases are generated so that the answer can fit in a <strong>32-bit</strong> integer.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [1,2,3], target = 4
+<strong>Output:</strong> 7
+<strong>Explanation:</strong>
+The possible combination ways are:
+(1, 1, 1, 1)
+(1, 1, 2)
+(1, 2, 1)
+(1, 3)
+(2, 1, 1)
+(2, 2)
+(3, 1)
+Note that different sequences are counted as different combinations.
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [9], target = 3
+<strong>Output:</strong> 0
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 200</code></li>
+	<li><code>1 &lt;= nums[i] &lt;= 1000</code></li>
+	<li>All the elements of <code>nums</code> are <strong>unique</strong>.</li>
+	<li><code>1 &lt;= target &lt;= 1000</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Follow up:</strong> What if negative numbers are allowed in the given array? How does it change the problem? What limitation we need to add to the question to allow negative numbers?</p>