Given an integer array <code>nums</code> with all positive numbers and no duplicates, find the number of possible combinations that add up to a positive integer <code>target</code>.
The different sequences are counted as different combinations.
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Example
Given nums = <code>[1, 2, 4]</code>, target = <code>4</code>
return <code>6</code>
解法一：
解法二：