function commonParentNode(oNode1, oNode2) {
for(; oNode1; oNode1 = oNode1.parentNode){
if(oNode1.contains(oNode2)){
return oNode1;
}
}
}
for有三个参数for(A;B;C)A可以省略不写，B是每一次循环需要满足的条件，B>0为真，B=0为假，C是每一次循环后做的操作，这里是让oNode1等于oNode1的父节点