电话目录管理系统

class PhoneDirectory {

    /** Initialize your data structure here
        @param maxNumbers - The maximum numbers that can be stored in the phone directory. */
    public PhoneDirectory(int maxNumbers) {
        ListNode head=new ListNode(0); 
        this.head=head;
        for(int i=1;i<maxNumbers;i++){
            head.next=new ListNode(i); 
            head=head.next;
        }
    }
    
    private ListNode head;
    
    /** Provide a number which is not assigned to anyone.
        @return - Return an available number. Return -1 if none is available. */
    public int get() {
        ListNode itr=head;
        while(itr!=null){
            if(!itr.use){
                itr.use=true;
                return itr.val;
            }
            itr=itr.next;
        }
        return -1;
    }
    
    /** Check if a number is available or not. */
    public boolean check(int number) {
        ListNode itr=head;
        while(itr!=null){
            if(itr.val==number){
                return !itr.use;
            }
            itr=itr.next;
        }
        return false;
    }
    
    /** Recycle or release a number. */
    public void release(int number) {
        ListNode itr=head;
        while(itr!=null){
            if(itr.val==number){
                itr.use=false;
                break;
            }
            itr=itr.next;
        }
    }
    
public static class ListNode {
    private int val;
    private ListNode next;
    private Boolean use=false;

    public ListNode(int val) {
      this.val = val;
    }
  }
}

/**
 * Your PhoneDirectory object will be instantiated and called as such:
 * PhoneDirectory obj = new PhoneDirectory(maxNumbers);
 * int param_1 = obj.get();
 * boolean param_2 = obj.check(number);
 * obj.release(number);
 */