答案:a.痕迹有:被害人露出的一双脚、被害人和作案者留下的鞋印、被害人的一只包和一只鞋、作案的匕首及一串滴漏在地上的油漆等,详见附图画圈处;b.真正的目击者只有李先生一人。分析如下:王先生说头上挨了一棍,可作案者丢弃在现场的凶器是匕首;楼先生用手机拍下的那家伙鞋底的纹路与案发现场留下的鞋印不符; The traces are: a pair of feet exposed by the victim, the shoemarks left by the victim and the perpetrator, a bag and a shoe of the victim, a dagger committing the crime, and a string of paint dripping on the ground, as shown in the attached picture Circle; b. The real eyewitness only Mr. Li one. Analysis is as follows: Mr. Wang said the head endured a stick, can be perpetrators discarded at the scene of the weapon is a dagger; floor with a cell phone photographed the texture of the soles of the shoe and the shoe scene does not match the shoe;