multiselect example using set and map

Requirement  : Mutliselect PickList Implementation using set and list concept

public  class MultiSelectExample {

     public List<String> cities{set;get;}

     public Set<String> nscities{set;get;}

     public set<String> scities{set;get;}

     public List<SelectOption> nsoptions{set;get;}

     public List<SelectOption> soptions{set;get;}

     public List<String> selected{set;get;}

     public List<String> removed{Set;get;}

     public MultiSelectExample(){

          cities=new List<String>{'Hyd','Ban','Che','Pune'};

          nscities=new Set<String>();

          scities=new  Set<String>();

          nscities.addAll(cities);

           nsoptions=new List<SelectOption>();

           soptions=new List<selectOption>();

           selectOption op=new SelectOption('none','-None-');

           soptions.add(op);

           createList();

     }

     public void createList(){

          nsoptions.clear();

          soptions.clear();

           selectOption op=new SelectOption('none','-None-');

           soptions.add(op);

           nsoptions.add(op);

          for(String s1: nscities){

              Selectoption op1=new SelectOption(s1,s1);

              nsoptions.add(op1);

          }

          for(String s2: scities){

              Selectoption op2=new SelectOption(s2,s2);

              soptions.add(op2);

          }

     }

     public void addEle(){

          nscities.removeAll(selected);

          scities.addAll(selected);

          createList();

     }

     public void removeEle(){

          scities.removeAll(removed);

          nscities.addAll(removed);

          createList();

     }   

}

VF page :

<apex:page controller="MultiSelectExample">

     <apex:form>

          <apex:pageBlock  id="one">

              <apex:panelGrid columns="3">

                   <apex:selectList value="{!selected}" multiselect="true" style="width:80px;height:80px;" >

                        <apex:selectOptions value="{!nsoptions}" />

                   </apex:selectList>

                   <apex:panelGrid columns="1">

                        <apex:commandButton value="Add"  action="{!addEle}" reRender="one"/>

                        <br/><br/>

                        <apex:commandButton value="Del"  action="{!removeEle}" reRender="one"/>

                   </apex:panelGrid>

                   <apex:selectList value="{!removed}" multiselect="true" style="width:80px;height:80px;" >

                        <apex:selectOptions value="{!soptions}" />

                   </apex:selectList>

              </apex:panelGrid>

          </apex:pageBlock>

     </apex:form>

</apex:page>