数组的三种声明方式：
1.var cars=new Array();
cars[0]="a",
cars[1]="b" ,
cars[2]="c";
2.var cars=new Array("a","b","c");
3.var cars=["a","b","c"]
注：整型不加引号；双引号内用单引号；注意英文输入法及大小写；
