链表继承自AbstractList
AbstractList implements自 List 接口

AbstractList

public abstract class AbstractList<E> implements List<E> {
    /**
     * 元素的数量
     */
    protected int size;

    /**
     * 元素的数量
     *
     * @return
     */
    public int size() {
        return size;
    }

    /**
     * 是否为空
     *
     * @return
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 是否包含某个元素
     *
     * @param element
     * @return
     */
    public boolean contains(E element) {
        return indexOf(element) != ELEMENT_NOT_FOUND;
    }

    /**
     * 添加元素到尾部
     *
     * @param element
     */
    public void add(E element) {
        add(size, element);
    }

    protected void outOfBounds(int index) {
        throw new IndexOutOfBoundsException("Index:" + index + ", Size:" + size);
    }

    protected void rangeCheck(int index) {
        if (index < 0 || index >= size) {
            outOfBounds(index);
        }
    }

    protected void rangeCheckForAdd(int index) {
        if (index < 0 || index > size) {
            outOfBounds(index);
        }
    }
}

List

public interface List<E> {
    static final int ELEMENT_NOT_FOUND = -1;

    /**
     * 清除所有元素
     */
    void clear();

    /**
     * 元素的数量
     *
     * @return
     */
    int size();

    /**
     * 是否为空
     *
     * @return
     */
    boolean isEmpty();

    /**
     * 是否包含某个元素
     *
     * @param element
     * @return
     */
    boolean contains(E element);

    /**
     * 添加元素到尾部
     *
     * @param element
     */
    void add(E element);

    /**
     * 获取index位置的元素
     *
     * @param index
     * @return
     */
    E get(int index);

    /**
     * 设置index位置的元素
     *
     * @param index
     * @param element
     * @return 原来的元素ֵ
     */
    E set(int index, E element);

    /**
     * 在index位置插入一个元素
     *
     * @param index
     * @param element
     */
    void add(int index, E element);

    /**
     * 删除index位置的元素
     *
     * @param index
     * @return
     */
    E remove(int index);

    /**
     * 查看元素的索引
     *
     * @param element
     * @return
     */
    int indexOf(E element);
}

单向链表

public class SingleLinkedList<E> extends AbstractList<E> {
    private Node<E> first;

    private static class Node<E> {
        E element;
        Node<E> next;

        public Node(E element, Node<E> next) {
            this.element = element;
            this.next = next;
        }
    }

    @Override
    public void clear() {
        size = 0;
        first = null;
    }

    @Override
    public E get(int index) {
        /*
         * 最好：O(1)
         * 最坏：O(n)
         * 平均：O(n)
         */
        return node(index).element;
    }

    @Override
    public E set(int index, E element) {
        /*
         * 最好：O(1)
         * 最坏：O(n)
         * 平均：O(n)
         */
        Node<E> node = node(index);
        E old = node.element;
        node.element = element;
        return old;
    }

    @Override
    public void add(int index, E element) {
        /*
         * 最好：O(1)
         * 最坏：O(n)
         * 平均：O(n)
         */
        rangeCheckForAdd(index);

        if (index == 0) {
            first = new Node<>(element, first);
        } else {
            Node<E> prev = node(index - 1);
            prev.next = new Node<>(element, prev.next);
        }
        size++;
    }

    @Override
    public E remove(int index) {
        /*
         * 最好：O(1)
         * 最坏：O(n)
         * 平均：O(n)
         */
        rangeCheck(index);

        Node<E> node = first;
        if (index == 0) {
            first = first.next;
        } else {
            Node<E> prev = node(index - 1);
            prev.next = prev.next.next;
        }
        size--;
        return node.element;
    }

    @Override
    public int indexOf(E element) {
        if (element == null) {
            Node<E> node = first;
            for (int i = 0; i < size; i++) {
                if (node.element == null) return i;

                node = node.next;
            }
        } else {
            Node<E> node = first;
            for (int i = 0; i < size; i++) {
                if (element.equals(node.element)) return i;

                node = node.next;
            }
        }
        return ELEMENT_NOT_FOUND;
    }

    /**
     * 获取index位置对应的节点对象
     *
     * @param index
     * @return
     */
    private Node<E> node(int index) {
        rangeCheck(index);

        Node<E> node = first;
        for (int i = 0; i < index; i++) {
            node = node.next;
        }
        return node;
    }

    @Override
    public String toString() {
        StringBuilder string = new StringBuilder();
        string.append("size=").append(size).append(", [");
        Node<E> node = first;
        for (int i = 0; i < size; i++) {
            if (i != 0) {
                string.append(", ");
            }

            string.append(node.element);

            node = node.next;
        }
        string.append("]");
        return string.toString();
    }
}

带虚拟节点的单向链表

有时候为了让代码更加精简,统一所有节点的处理逻辑,可以在最前面增加一个虚拟的头结点(不存储数据)

/**
 * 增加一个虚拟头结点
 * @author MJ Lee
 *
 * @param <E>
 */
public class SingleLinkedList2<E> extends AbstractList<E> {
	private Node<E> first;
	
	public SingleLinkedList2() {
		first = new Node<>(null, null);
	}
	
	private static class Node<E> {
		E element;
		Node<E> next;
		public Node(E element, Node<E> next) {
			this.element = element;
			this.next = next;
		}
	}

	@Override
	public void clear() {
		size = 0;
		first = null;
	}

	@Override
	public E get(int index) {
		return node(index).element;
	}

	@Override
	public E set(int index, E element) {
		Node<E> node = node(index);
		E old = node.element;
		node.element = element;
		return old;
	}

	@Override
	public void add(int index, E element) {
		rangeCheckForAdd(index);
		
		Node<E> prev = index == 0 ? first : node(index - 1);
		prev.next = new Node<>(element, prev.next);

		size++;
	}

	@Override
	public E remove(int index) {
		rangeCheck(index);
		
		Node<E> prev = index == 0 ? first : node(index - 1);
		Node<E> node = prev.next;
		prev.next = node.next;
			
		size--;
		return node.element;
	}

	@Override
	public int indexOf(E element) {
		if (element == null) {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (node.element == null) return i;
				
				node = node.next;
			}
		} else {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (element.equals(node.element)) return i;
				
				node = node.next;
			}
		}
		return ELEMENT_NOT_FOUND;
	}
	
	/**
	 * 获取index位置对应的节点对象
	 * @param index
	 * @return
	 */
	private Node<E> node(int index) {
		rangeCheck(index);
		
		Node<E> node = first.next;
		for (int i = 0; i < index; i++) {
			node = node.next;
		}
		return node;
	}
	
	@Override
	public String toString() {
		StringBuilder string = new StringBuilder();
		string.append("size=").append(size).append(", [");
		Node<E> node = first.next;
		for (int i = 0; i < size; i++) {
			if (i != 0) {
				string.append(", ");
			}
			
			string.append(node.element);
			
			node = node.next;
		}
		string.append("]");
		return string.toString();
	}
}

题目一：删除链表中的节点

    /**
     * 利用取巧的方式删除节点，将要删除的节点的下一个节点的值覆盖到要删除的节点上，之后删除下一个节点
     *
     * @param node
     */
    public void remove(Node node) {
        node.element = node.next.element;
        node.next = node.next.next;
    }

题目二：反转链表

解法一：递归

public ListNode reverseList(ListNode head) {
    if (head == null || head.next == null) return head;

    ListNode newHead = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return newHead;
}

解法二：非递归

public ListNode reverseList2(ListNode head) {
    if (head == null || head.next == null) return head;

    ListNode newHead = null;
    while (head != null) {
        ListNode tmp = head.next;
        head.next = newHead;
        newHead = head;
        head = tmp;
    }
    return newHead;
}

题目三：判断是否有环（快慢指针）

public boolean hasLoop(ListNode head) {
    if (head == null || head.next == null) return false;

    ListNode slow = head;
    ListNode fast = head.next;
    while (fast != null && fast.next != null) {
        slow = slow.next;
        fast = fast.next.next;

        if (slow == fast) return true;
    }
    return false;
}

双向链表

public class LinkedList<E> extends AbstractList<E> {
    private Node<E> first;
    private Node<E> last;

    private static class Node<E> {
        E element;
        Node<E> prev;
        Node<E> next;

        public Node(Node<E> prev, E element, Node<E> next) {
            this.prev = prev;
            this.element = element;
            this.next = next;
        }

        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder();

            if (prev != null) {
                sb.append(prev.element);
            } else {
                sb.append("null");
            }

            sb.append("_").append(element).append("_");

            if (next != null) {
                sb.append(next.element);
            } else {
                sb.append("null");
            }

            return sb.toString();
        }
    }

    @Override
    public void clear() {
        size = 0;
        first = null;
        last = null;
    }

    @Override
    public E get(int index) {
        return node(index).element;
    }

    @Override
    public E set(int index, E element) {
        Node<E> node = node(index);
        E old = node.element;
        node.element = element;
        return old;
    }

    @Override
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        // size == 0
        // index == 0
        if (index == size) { // 往最后面添加元素
            Node<E> oldLast = last;
            last = new Node<>(oldLast, element, null);
            if (oldLast == null) { // 这是链表添加的第一个元素
                first = last;
            } else {
                oldLast.next = last;
            }
        } else {
            Node<E> next = node(index);
            Node<E> prev = next.prev;
            Node<E> node = new Node<>(prev, element, next);
            next.prev = node;

            if (prev == null) { // index == 0
                first = node;
            } else {
                prev.next = node;
            }
        }

        size++;
    }

    @Override
    public E remove(int index) {
        rangeCheck(index);

        Node<E> node = node(index);
        Node<E> prev = node.prev;
        Node<E> next = node.next;

        if (prev == null) { // index == 0
            first = next;
        } else {
            prev.next = next;
        }

        if (next == null) { // index == size - 1
            last = prev;
        } else {
            next.prev = prev;
        }

        size--;
        return node.element;
    }

    @Override
    public int indexOf(E element) {
        if (element == null) {
            Node<E> node = first;
            for (int i = 0; i < size; i++) {
                if (node.element == null) return i;

                node = node.next;
            }
        } else {
            Node<E> node = first;
            for (int i = 0; i < size; i++) {
                if (element.equals(node.element)) return i;

                node = node.next;
            }
        }
        return ELEMENT_NOT_FOUND;
    }

    /**
     * 获取index位置对应的节点对象
     *
     * @param index
     * @return
     */
    private Node<E> node(int index) {
        rangeCheck(index);

        if (index < (size >> 1)) {
            Node<E> node = first;
            for (int i = 0; i < index; i++) {
                node = node.next;
            }
            return node;
        } else {
            Node<E> node = last;
            for (int i = size - 1; i > index; i--) {
                node = node.prev;
            }
            return node;
        }
    }

    @Override
    public String toString() {
        StringBuilder string = new StringBuilder();
        string.append("size=").append(size).append(", [");
        Node<E> node = first;
        for (int i = 0; i < size; i++) {
            if (i != 0) {
                string.append(", ");
            }

            string.append(node);

            node = node.next;
        }
        string.append("]");
        return string.toString();
    }
}

动态数组VS双向链表

动态数组: 开辟、销毁内存空间的次数相对较少,但可能造成内存空间浪费(可以通过缩容解决)
双向链表: 开辟、销毁内存空间的次数相对较多,但不会造成内存空间的浪费

如果频繁在尾部进行添加、删除操作,动态数组、双向链表均可选择
如果频繁在头部进行添加、删除操作,建议选择使用双向链表
如果有频繁的**(在任意位置)添加、删除操作,建议选择使用双向链表**
如果有频繁的查询操作(随机访问操作),建议选择使用动态数组

循环单向链表

public class SingleCircleLinkedList<E> extends AbstractList<E> {
	private Node<E> first;
	
	private static class Node<E> {
		E element;
		Node<E> next;
		public Node(E element, Node<E> next) {
			this.element = element;
			this.next = next;
		}
		
		@Override
		public String toString() {
			StringBuilder sb = new StringBuilder();
			sb.append(element).append("_").append(next.element);
			return sb.toString();
		}
	}

	@Override
	public void clear() {
		size = 0;
		first = null;
	}

	@Override
	public E get(int index) {
		return node(index).element;
	}

	@Override
	public E set(int index, E element) {
		Node<E> node = node(index);
		E old = node.element;
		node.element = element;
		return old;
	}

	@Override
	public void add(int index, E element) {
		rangeCheckForAdd(index);
		
		if (index == 0) {
			Node<E> newFirst = new Node<>(element, first);
			// 拿到最后一个节点
			Node<E> last = (size == 0) ? newFirst : node(size - 1);
			last.next = newFirst;
			first = newFirst;
		} else {
			Node<E> prev = node(index - 1);
			prev.next = new Node<>(element, prev.next);
		}
		size++;
	}

	@Override
	public E remove(int index) {
		rangeCheck(index);
		
		Node<E> node = first;
		if (index == 0) {
			if (size == 1) {
				first = null;
			} else {
				Node<E> last = node(size - 1);
				first = first.next;
				last.next = first;
			}
		} else {
			Node<E> prev = node(index - 1);
			node = prev.next;
			prev.next = node.next;
		}
		size--;
		return node.element;
	}

	@Override
	public int indexOf(E element) {
		if (element == null) {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (node.element == null) return i;
				
				node = node.next;
			}
		} else {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (element.equals(node.element)) return i;
				
				node = node.next;
			}
		}
		return ELEMENT_NOT_FOUND;
	}
	
	/**
	 * 获取index位置对应的节点对象
	 * @param index
	 * @return
	 */
	private Node<E> node(int index) {
		rangeCheck(index);
		
		Node<E> node = first;
		for (int i = 0; i < index; i++) {
			node = node.next;
		}
		
		return node;
	}
	
	@Override
	public String toString() {
		StringBuilder string = new StringBuilder();
		string.append("size=").append(size).append(", [");
		Node<E> node = first;
		for (int i = 0; i < size; i++) {
			if (i != 0) {
				string.append(", ");
			}
			
			string.append(node);
			
			node = node.next;
		}
		string.append("]");
		return string.toString();
	}
}

循环双向链表

public class CircleLinkedList<E> extends AbstractList<E> {
	private Node<E> first;
	private Node<E> last;
	private Node<E> current; 
	
	private static class Node<E> {
		E element;
		Node<E> prev;
		Node<E> next;
		public Node(Node<E> prev, E element, Node<E> next) {
			this.prev = prev;
			this.element = element;
			this.next = next;
		}
		
		@Override
		public String toString() {
			StringBuilder sb = new StringBuilder();
			
			if (prev != null) {
				sb.append(prev.element);
			} else {
				sb.append("null");
			}
			
			sb.append("_").append(element).append("_");

			if (next != null) {
				sb.append(next.element);
			} else {
				sb.append("null");
			}
			
			return sb.toString();
		}
	}
	
	public void reset() {
		current = first;
	}
	
	public E next() {
		if (current == null) return null;
		
		current = current.next;
		return current.element;
	}
	
	public E remove() {
		if (current == null) return null;
		
		Node<E> next = current.next; 
		E element = remove(current);
		if (size == 0) {
			current = null;
		} else {
			current = next;
		}
		
		return element;
	}

	@Override
	public void clear() {
		size = 0;
		first = null;
		last = null;
	}

	@Override
	public E get(int index) {
		return node(index).element;
	}

	@Override
	public E set(int index, E element) {
		Node<E> node = node(index);
		E old = node.element;
		node.element = element;
		return old;
	}

	@Override
	public void add(int index, E element) {
		rangeCheckForAdd(index);

		// size == 0
		// index == 0
		if (index == size) { // 往最后面添加元素
			Node<E> oldLast = last;
			last = new Node<>(oldLast, element, first);
			if (oldLast == null) { // 这是链表添加的第一个元素
				first = last;
				first.next = first;
				first.prev = first;
			} else {
				oldLast.next = last;
				first.prev = last;
			}
		} else {
			Node<E> next = node(index); 
			Node<E> prev = next.prev; 
			Node<E> node = new Node<>(prev, element, next);
			next.prev = node;
			prev.next = node;
			
			if (next == first) { // index == 0
				first = node;
			}
		}
		
		size++;
	}

	@Override
	public E remove(int index) {
		rangeCheck(index);
		return remove(node(index));
	}
	
	private E remove(Node<E> node) {
		if (size == 1) {
			first = null;
			last = null;
		} else {
			Node<E> prev = node.prev;
			Node<E> next = node.next;
			prev.next = next;
			next.prev = prev;
			
			if (node == first) { // index == 0
				first = next;
			}
			
			if (node == last) { // index == size - 1
				last = prev;
			}
		}
		
		size--;
		return node.element;
	}

	@Override
	public int indexOf(E element) {
		if (element == null) {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (node.element == null) return i;
				
				node = node.next;
			}
		} else {
			Node<E> node = first;
			for (int i = 0; i < size; i++) {
				if (element.equals(node.element)) return i;
				
				node = node.next;
			}
		}
		return ELEMENT_NOT_FOUND;
	}
	
	/**
	 * 获取index位置对应的节点对象
	 * @param index
	 * @return
	 */
	private Node<E> node(int index) {
		rangeCheck(index);
		
		if (index < (size >> 1)) {
			Node<E> node = first;
			for (int i = 0; i < index; i++) {
				node = node.next;
			}
			return node;
		} else {
			Node<E> node = last;
			for (int i = size - 1; i > index; i--) {
				node = node.prev;
			}
			return node;
		}
	}
	
	@Override
	public String toString() {
		StringBuilder string = new StringBuilder();
		string.append("size=").append(size).append(", [");
		Node<E> node = first;
		for (int i = 0; i < size; i++) {
			if (i != 0) {
				string.append(", ");
			}
			
			string.append(node);
			
			node = node.next;
		}
		string.append("]");
		return string.toString();
	}
}