/**
@author: Mathias Van Malderen (tux4life)
*/
void mystrcat(char * s1, const char * s2)
{
    while(*s1) s1++;
    for(; *s1 = *s2; s1++, s2++);
}

Or you could rewrite it in a shorter way, thanks to William Hemsworth

void mystrcat(char * s1, const char * s2)
{
    while(*s1++);
    while(*s1++ = *s2++);
}
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