var a = ["1","3","5","9","7"];

var b =["3","1","5","7","9"];

1.先判断数组的长度

2.如果长度相等，再将数组排序用sort();

3.将两数组转换为string再进行比较（因为a,b的存储位置不同）

JSON.stringify(a) == JSON.stringify(b);  //true