Lexeme pollFirst(){
		if(this.size == 1){
			Lexeme first = this.head.lexeme;
			this.head = null;
			this.tail = null;
			this.size--;
			return first;
		}else if(this.size > 1){
			Lexeme first = this.head.lexeme;
			this.head = this.head.next;
			this.size --;
			return first;
		}else{
			return null;
		}
	}
Lexeme pollFirst(){
		if(this.size == 1){
			Lexeme first = this.head.lexeme;
			this.head = null;
			this.tail = null;
			this.size--;
			return first;
		}else if(this.size > 1){
			Lexeme first = this.head.lexeme;
			this.head = this.head.next;
			this.size --;
			return first;
		}else{
			return null;
		}
	}
Lexeme pollFirst ( ) { if ( this . size == 1 ) { Lexeme first = this . head . lexeme ; this . head = null ; this . tail = null ; this . size -- ; return first ; } else if ( this . size > 1 ) { Lexeme first = this . head . lexeme ; this . head = this . head . next ; this . size -- ; return first ; } else { return null ; } }
取出链表集合的第一个元素
@return Lexeme
取出链表集合的第一个元素