LinkedList与链表

1. ArrayList的缺陷

当在ArrayList任意位置插入或者删除元素时，就需要将后序元素整体往前或者往后搬移，时间复杂度为O(n)，效率比较低，因此ArrayList不适合做任意位置插入和删除比较多的场景。因此：java 集合中又引入了LinkedList，即链表结构。

2. 链表

2.1 链表的概念及结构

链表是一种物理存储结构上非连续存储结构，数据元素的逻辑顺序是通过链表中的引用链接次序实现的 。

2.2 链表的实现

1.头插法 addFirst()

public void addFirst(int data) {
        ListNode node = new ListNode(data);
        node.next = head;
        head = node;
    }

2.尾插法 addLast()

public void addList(int data) {
        ListNode node = new ListNode(data);
        if (head == null) {
            head = node;
        }
        ListNode cur = head;
        while (cur.next != null) {
            cur = cur.next;
        }
        cur.next = node;
    }

3.addIndex()

public void addIndex(int index, int data) {
        int len = size();
        // 不合法
        if (index < 0 || index > len) {
            System.out.println("index不合法");
            return;
        }
        // 空链表
        if (index == 0) {
            addFirst(data);
            return;
        }
        // 尾部插入
        if (index == len) {
            addLast(data);
            return;
        }

        //中间位置插入
        ListNode cur = head;
        while (index - 1 != 0) {
            cur = cur.next;
            index--;
        }
        ListNode node = new ListNode(data);
        node.next = cur.next;
        cur.next = node;
    }

4.contains()

public boolean contains(int key) {
        ListNode cur = head;
        while (cur != null) {
            if (cur.val == key) {
                return true;
            }
            cur = cur.next;
        }
        return false;
    }

5.remove()

public void remove(int key) {
        if (head == null) {
            return;
        }
        if (head.val == key) {
            head = head.next;
        }

        ListNode cur = findNodeOfKey(key);
        if (cur == null) {
            return;
        }
        ListNode del = cur.next;
        cur.next = del.next;
    }

    private ListNode findNodeOfKey(int key) {
        ListNode cur = head;
        while (cur.next != null) {
            if (cur.next.val == key) {
                return cur;
            }
            cur = cur.next;
        }
        return null;
    }

6.removeAllKey()

public void removeAllKey(int key) {
        ListNode cur = head.next;
        ListNode prev = head;
        while (cur != null) {
            if (cur.val == key) {
                prev.next = cur.next;
            } else {
                prev = cur;
            }
            cur = cur.next;
        }
        if (head.val == key) {
            head = head.next;
        }
    }

7.clear()

 public void clear() {
        ListNode cur = head;
        while (cur != null) {
            ListNode curNext = cur.next;
            cur.next = null;
            cur = curNext;
        }
        head = null;
    }

 8.size()

public int size() {
        int count = 0;
        ListNode cur = head;
        while (cur != null) {
            count++;
            cur = cur.next;
        }
        return count;
    }

9.display()

public void display() {
        ListNode cur = head;
        while (cur != null) {
            System.out.print(cur.val + " ");
            cur = cur.next;
        }
    }