var Ptr=document.getElementById("tab").getElementsByTagName("tr");
function $() {
    for (i=1;i<Ptr.length+1;i++) { 
    Ptr[i-1].className = (i%2>0)?"t1":"t2"; 
    }
}
window.onload=$;
for(var i=0;i<Ptr.length;i++) {
    Ptr[i].onmouseover=function(){
    this.tmpClass=this.className;
    this.className = "t3";
    
    };
    Ptr[i].onmouseout=function(){
    this.className=this.tmpClass;
    };
}