Lexeme pollLast(){
		if(this.size == 1){
			Lexeme last = this.head.lexeme;
			this.head = null;
			this.tail = null;
			this.size--;
			return last;
			
		}else if(this.size > 1){
			Lexeme last = this.tail.lexeme;
			this.tail = this.tail.prev;
			this.size--;
			return last;
			
		}else{
			return null;
		}
	}
Lexeme pollLast(){
		if(this.size == 1){
			Lexeme last = this.head.lexeme;
			this.head = null;
			this.tail = null;
			this.size--;
			return last;
			
		}else if(this.size > 1){
			Lexeme last = this.tail.lexeme;
			this.tail = this.tail.prev;
			this.size--;
			return last;
			
		}else{
			return null;
		}
	}
Lexeme pollLast ( ) { if ( this . size == 1 ) { Lexeme last = this . head . lexeme ; this . head = null ; this . tail = null ; this . size -- ; return last ; } else if ( this . size > 1 ) { Lexeme last = this . tail . lexeme ; this . tail = this . tail . prev ; this . size -- ; return last ; } else { return null ; } }
取出链表集合的最后一个元素
@return Lexeme
取出链表集合的最后一个元素