手写链表~内含单向链表和双向链表，请大家放心食用

力扣707题

单向链表

public class ListNode {
    
    int val;
    ListNode next;
    ListNode(int x) {
     
        val = x;
    }
}

class MyLinkedList {
    

    int size;
    ListNode head;

    public MyLinkedList() {
    
        size = 0;
        head = new ListNode(0);
    }
    
    public int get(int index) {
    
        //无效索引，返回-1
        if(index < 0 || index >= size) {
    
            return -1;
        }
        //寻找节点值
        ListNode cur = head;
        for(int i = 0; i < index + 1; i++) {
    
            cur = cur.next;
        }
        return cur.val;
    }
    
    public void addAtHead(int val) {
    
        //相当于在索引为0的链表处添加了一个节点
        addAtIndex(0, val);
    }
    
    public void addAtTail(int val) {
    
        //相当于在索引为size的链表处添加了一个节点
        addAtIndex(size, val);
    }
    
    public void addAtIndex(int index, int val) {
    
        //添加节点位置 大于 链表索引
        if(index > size) {
    
            return;
        }
        //添加索引 小于 0， 那就给它算作是0
        if(index < 0) {
    
            index = 0;
        }
        size++;
        //寻找要添加索引的前一个 节点
        ListNode pre = head;
        for(int i = 0; i < index; i++) {
    
            pre = pre.next;
        }
        //添加节点
        ListNode toAdd = new ListNode(val);
        toAdd.next = pre.next;
        pre.next = toAdd;
    }
    
    public void deleteAtIndex(int index) {
    
        //无效索引
        if(index < 0 || index >= size) {
    
            return;
        }
        size--;
        //寻找要删除节点的前一个结点
        ListNode pre = head;
        for(int i = 0; i < index; i++) {
    
            pre = pre.next;
        }
        //删除操作
        pre.next = pre.next.next;
    }

}

双向链表

public class ListNode {
    
    int val;
    ListNode next;
    ListNode pre;
    ListNode(int x) {
     
        val = x;
    }
}

class MyLinkedList {
    

    int size;
    ListNode head;
    ListNode tail;

    public MyLinkedList() {
    
        size = 0;
        head = new ListNode(0);
        tail = new ListNode(0);
        head.next = tail;
        tail.pre = head;
    }
    
    public int get(int index) {
    
        //无效索引，返回-1
        if(index < 0 || index >= size) {
    
            return -1;
        }
        //寻找节点值
        ListNode cur = head;
        if (index + 1 < size - index) {
    
            for(int i = 0; i < index + 1; ++i) {
    
                cur = cur.next;
            }
        } else {
    
            cur = tail;
            for(int i = 0; i < size - index; ++i) {
    
                cur = cur.pre;
            } 
        }
        return cur.val;
    }
    
    public void addAtHead(int val) {
    
        ListNode pre = head;
        ListNode after = head.next;
        size++;
        //添加节点
        ListNode toAdd = new ListNode(val);
        toAdd.next = after;
        toAdd.pre = pre;
        pre.next = toAdd;
        after.pre = toAdd;
    }
    
    public void addAtTail(int val) {
    
        ListNode pre = tail.pre;
        ListNode after = tail;
        size++;
        //添加节点
        ListNode toAdd = new ListNode(val);
        toAdd.next = after;
        toAdd.pre = pre;
        pre.next = toAdd;
        after.pre = toAdd;
    }
    
    public void addAtIndex(int index, int val) {
    
        //添加节点位置 大于 链表索引
        if(index > size) {
    
            return;
        }
        //添加索引 小于 0， 那就给它算作是0
        if(index < 0) {
    
            index = 0;
        }
        //寻找要添加索引的前一个节点 和 后一个节点
        ListNode pre, after;
        if(index < size - index) {
    
            pre = head;
            for(int i = 0; i < index; i++) {
    
                pre = pre.next;
            }
            after = pre.next;
        } else {
    
            after = tail;
            for(int i = 0; i < size - index; i++) {
    
                after = after.pre;
            }
            pre = after.pre;
        }
        size++;
        //添加节点
        ListNode toAdd = new ListNode(val);
        toAdd.next = after;
        toAdd.pre = pre;
        pre.next = toAdd;
        after.pre = toAdd;
    }
    
    public void deleteAtIndex(int index) {
    
        //无效索引
        if(index < 0 || index >= size) {
    
            return;
        }
        //寻找要删除索引的前一个节点 和 后一个节点
        ListNode pre, after;
        if(index < size - index) {
    
            pre = head;
            for(int i = 0; i < index; i++) {
    
                pre = pre.next;
            }
            after = pre.next.next;
        } else {
    
            after = tail;
            //size - index - 1为啥？你得找到所要删除节点的前一个节点的索引，那必须减一
            for(int i = 0; i < size - index - 1; i++) {
    
                after = after.pre;
            }
            pre = after.pre.pre;
        }
        size--;
        //删除操作
        pre.next = after;
        after.pre = pre;
    }

}



/** * Your MyLinkedList object will be instantiated and called as such: * MyLinkedList obj = new MyLinkedList(); * int param_1 = obj.get(index); * obj.addAtHead(val); * obj.addAtTail(val); * obj.addAtIndex(index,val); * obj.deleteAtIndex(index); */

双向链表也可以不用这么写，不考虑是从前还是从后开始查，这样会更简单一些，哈哈！那这样的话，代码比我这样写要简单很多了，只需要考虑从前往后查就可以！