用javascript简单实现省市区级联菜单
1、先列举出名为city.xml的的部分省市区

<?xml version="1.0" encoding="UTF-8"?>

<root name="中国">

<province name="请选择省" postcode="100000" >

    <city name="请选择市" postcode="100100" >

  <area name="请选择区" postcode="100101" />

</city>

  </province>

  <province name="北京市" postcode="110000" >

    <city name="市辖区" postcode="110100" >

        <area name="东城区" postcode="110101" />

        <area name="西城区" postcode="110102" />

        <area name="崇文区" postcode="110103" />

        <area name="宣武区" postcode="110104" />

        <area name="朝阳区" postcode="110105" />

        <area name="丰台区" postcode="110106" />

        <area name="石景山区" postcode="110107" />

        <area name="海淀区" postcode="110108" />

        <area name="门头沟区" postcode="110109" />

        <area name="房山区" postcode="110111" />

        <area name="通州区" postcode="110112" />

        <area name="顺义区" postcode="110113" />

        <area name="昌平区" postcode="110114" />

        <area name="大兴区" postcode="110115" />

        <area name="怀柔区" postcode="110116" />

        <area name="平谷区" postcode="110117" />

    </city>

    <city name="县" postcode="110200" >

        <area name="密云县" postcode="110228" />

        <area name="延庆县" postcode="110229" />

    </city>

  </province>

  <province name="天津市" postcode="120000" >

    <city name="市辖区" postcode="120100" >

        <area name="和平区" postcode="120101" />

        <area name="河东区" postcode="120102" />

        <area name="河西区" postcode="120103" />

        <area name="南开区" postcode="120104" />

        <area name="河北区" postcode="120105" />

        <area name="红桥区" postcode="120106" />

        <area name="塘沽区" postcode="120107" />

        <area name="汉沽区" postcode="120108" />

        <area name="大港区" postcode="120109" />

        <area name="东丽区" postcode="120110" />

        <area name="西青区" postcode="120111" />

        <area name="津南区" postcode="120112" />

        <area name="北辰区" postcode="120113" />

        <area name="武清区" postcode="1