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copied!<p>In a single function you could write:</p> <pre><code>// pre: to is a string that is zero-initiallized (all chars are '\0') void mystrcat( char* to, const char* from ) { if ( *to ) mystrcat( to+1, from ); else { if (*to = *from) mystrcat( to+1, from+1 ); } } </code></pre> <p>Note that it is intentionally ugly, it serves to purposes in two branches, it advances the <code>to</code> pointer in the first branch and it copies values in the other. The if condition is intentionally ugly also as most usually you will flag a single = within a condition as a typo, but insist in that it is up, running and tested so that they must work the meaning out themselves.</p> <p>One other thing you might consider is adding a return statement like:</p> <pre><code>char* mystrcat2( char* to, const char* from ) { if ( *to ) return mystrcat2( to+1, from ); else { if ( *to = *from ) return mystrcat2( to+1, from+1 ); else return to; } } </code></pre> <p>Now the interviewed has the second task of interpreting the result of the function when called on two strings, and you can discuss how the efficiency of this <code>strcat</code> version compares to the original both in terms of single string concatenations as well as multiple concatenations (take a look at <a href="http://www.joelonsoftware.com/articles/fog0000000319.html" rel="nofollow noreferrer">this article</a> by Joel Spolsky that among other things talks about the Scheimel paint algorithm). You can ask for the implementation of this function in terms of both the <code>strcat</code> and <code>mystrcat</code>:</p> <pre><code>// concatenates a set of strings into a buffer // // pre: to has memory enough to hold all the contents // from[i] is a valid C string for all i smaller than n void mymultiplecat( char* to, const char* from[], int n ) { for ( int i = 0; i < n; ++i ) { to = mystrcat( to, from[i] ); } } </code></pre>

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