List<Schema.PicklistEntry> picklistValues = Lead.fields.Lead_City__c.getDescribe().getpicklistvalues();
set<string> LeadCityValues = new set<string>();
for(Schema.PicklistEntry value : picklistValues){
   LeadCityValues.add(value.getLabel());
}

for( Lead ld : Trigger.new ) {  if( ld.Lead_City__c != null && !(LeadCityValues.contains(ld.Lead_City__c)) ){
       ld.Other_Lead_City__c = ld.Lead_City__c;
       ld.Lead_City__c = 'Others';  } }