Yulia Efimova prepares for a head in the women's 100-meter breaststroke on Sunday.An unfamiliar sound echoed through the Olympic Aquatics Stadium on Sunday.Boos greeted Russia&rsquo;s Yulia Efimova during her preliminary heat of the 100-meter breaststroke, after a confusing two-week odyssey in which&nbsp;she was barred from the Olympics then quietly reinstated.Last month, the International Swimming Federation announced Efimova&rsquo;s Olympic ban as part of Russia&rsquo;s doping scandal. She was one of seven Russian swimmers barred from the Games who had either failed doping tests or were named in the World Anti-Doping Agency&rsquo;s investigation into state-sponsored doping.Efimova, 24, previously served a 16-month suspension for doping and earlier this year failed a test for meldonium, a banned substance, though the result was overturned.&ldquo;I don&rsquo;t know what to do,&rdquo; she said after recording the second-fastest preliminary time. &ldquo;It was crazy the last year and a half. I didn&rsquo;t understand what was going on. I&rsquo;m just happy to be here and ready to race.&rdquo;Efimova declined to answer further questions.She has trained with the USC-based Trojan Swim Club. Same with Vladimir Morozov and Nikita Lobintsev, who are among the once-banned Russians who have been added to Olympic start lists without explanation.American Lilly King, a first-time Olympian, recorded the top preliminary time in the event. The final is Monday.