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    <p>So, I see you want to implement a method: <code>subSequence(s, n)</code>: Which returns a collection of all character character <em>combinations</em> from <code>s</code> of length <code>n</code>, such that ordering is preserved.</p> <p>In the spirit of your desire to <strong>not</strong> provide you with code, I assume you would prefer no pseudo-code either. So, I will explain my suggested approach in a narrative fashion, leaving the translation to procedural code as an exercise-to-the-reader(TM).</p> <p>Think of this problem where you are obtaining all combinations of character <em>positions</em>, which could be represented as an array of bits (a.k.a. flags). So where <code>s="abcd"</code> and <code>n=3</code> (as in your example), all combinations could be represented as follows:</p> <pre><code>1110 = abc 1101 = abd 1011 = acd 0111 = bcd </code></pre> <p>Note, that we start with a bit-field where all characters are turned "on" and then shift the "off" bit over by 1. Things get interesting in an example where <code>n &lt; length(s) - 1</code>. For example, say <code>s="abcd"</code> and <code>n=2</code>. Then we have:</p> <pre><code>1100 = ab 1001 = ad 1010 = ac 0110 = bc 0101 = bd 0011 = cd </code></pre> <p>The recursion comes into play when you analyze a sub set of the bit-fields. Hence, a recursive call would reduce the size of the bit-field and "bottom-out" where you have three flags:</p> <pre><code>100 010 001 </code></pre> <p>The bulk of the work is a recursive approach to find all of the bit-fields. Once you have them, the positions of each bit can be used as an index in the the array of characters (that is <code>s</code>).</p> <p>This should be sufficient to get you started on some pseudo-code!</p>
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    1. USRyan Delucchi
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    1. COafter some of his comments, I realized he did care about the order of the substrings... so this solution is not what he wants... I edited my answer 'cause of that
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