707. Design Linked List

Description

Design your implementation of the linked list. You can choose to use a singly or doubly linked list.
A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node.
If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement the MyLinkedList class:

MyLinkedList() Initializes the MyLinkedList object.
int get(int index) Get the value of the index^th node in the linked list. If the index is invalid, return -1.
void addAtHead(int val) Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
void addAtTail(int val) Append a node of value val as the last element of the linked list.
void addAtIndex(int index, int val) Add a node of value val before the index^th node in the linked list. If index equals the length of the linked list, the node will be appended to the end of the linked list. If index is greater than the length, the node will not be inserted.
void deleteAtIndex(int index) Delete the index^th node in the linked list, if the index is valid.

Example 1:

Input
["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"]
[[], [1], [3], [1, 2], [1], [1], [1]]
Output
[null, null, null, null, 2, null, 3]

Explanation
MyLinkedList myLinkedList = new MyLinkedList();
myLinkedList.addAtHead(1);
myLinkedList.addAtTail(3);
myLinkedList.addAtIndex(1, 2);    // linked list becomes 1->2->3
myLinkedList.get(1);              // return 2
myLinkedList.deleteAtIndex(1);    // now the linked list is 1->3
myLinkedList.get(1);              // return 3

Constraints:

0 <= index, val <= 1000
Please do not use the built-in LinkedList library.
At most 2000 calls will be made to get, addAtHead, addAtTail, addAtIndex and deleteAtIndex.

Solutions

Solution 1

Thinking

We must implement index access, head/tail inserts, and indexed deletes without a library list. At most \(2000\) calls, so an \(O(n)\) walk from the head is fine.

A singly linked list can reach index \(\textit{index}\) only by walking predecessors. A dummy head unifies empty and non-empty updates; \(\textit{cnt}\) makes bounds checks independent of the walk.

Head and tail inserts reduce to \(\textit{addAtIndex}\): walk to the predecessor, then relink. Deletes do the same. Each operation is \(O(n)\).

Python3JavaC++GoTypeScriptRust

class MyLinkedList:
    def __init__(self):
        self.dummy = ListNode()
        self.cnt = 0

    def get(self, index: int) -> int:
        if index < 0 or index >= self.cnt:
            return -1
        cur = self.dummy.next
        for _ in range(index):
            cur = cur.next
        return cur.val

    def addAtHead(self, val: int) -> None:
        self.addAtIndex(0, val)

    def addAtTail(self, val: int) -> None:
        self.addAtIndex(self.cnt, val)

    def addAtIndex(self, index: int, val: int) -> None:
        if index > self.cnt:
            return
        pre = self.dummy
        for _ in range(index):
            pre = pre.next
        pre.next = ListNode(val, pre.next)
        self.cnt += 1

    def deleteAtIndex(self, index: int) -> None:
        if index >= self.cnt:
            return
        pre = self.dummy
        for _ in range(index):
            pre = pre.next
        t = pre.next
        pre.next = t.next
        t.next = None
        self.cnt -= 1


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

class MyLinkedList {
    private ListNode dummy = new ListNode();
    private int cnt;

    public MyLinkedList() {
    }

    public int get(int index) {
        if (index < 0 || index >= cnt) {
            return -1;
        }
        var cur = dummy.next;
        while (index-- > 0) {
            cur = cur.next;
        }
        return cur.val;
    }

    public void addAtHead(int val) {
        addAtIndex(0, val);
    }

    public void addAtTail(int val) {
        addAtIndex(cnt, val);
    }

    public void addAtIndex(int index, int val) {
        if (index > cnt) {
            return;
        }
        var pre = dummy;
        while (index-- > 0) {
            pre = pre.next;
        }
        pre.next = new ListNode(val, pre.next);
        ++cnt;
    }

    public void deleteAtIndex(int index) {
        if (index < 0 || index >= cnt) {
            return;
        }
        var pre = dummy;
        while (index-- > 0) {
            pre = pre.next;
        }
        var t = pre.next;
        pre.next = t.next;
        t.next = null;
        --cnt;
    }
}

/**
 * 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);
 */

class MyLinkedList {
private:
    ListNode* dummy = new ListNode();
    int cnt = 0;

public:
    MyLinkedList() {
    }

    int get(int index) {
        if (index < 0 || index >= cnt) {
            return -1;
        }
        auto cur = dummy->next;
        while (index--) {
            cur = cur->next;
        }
        return cur->val;
    }

    void addAtHead(int val) {
        addAtIndex(0, val);
    }

    void addAtTail(int val) {
        addAtIndex(cnt, val);
    }

    void addAtIndex(int index, int val) {
        if (index > cnt) {
            return;
        }
        auto pre = dummy;
        while (index-- > 0) {
            pre = pre->next;
        }
        pre->next = new ListNode(val, pre->next);
        ++cnt;
    }

    void deleteAtIndex(int index) {
        if (index >= cnt) {
            return;
        }
        auto pre = dummy;
        while (index-- > 0) {
            pre = pre->next;
        }
        auto t = pre->next;
        pre->next = t->next;
        t->next = nullptr;
        --cnt;
    }
};

/**
 * 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);
 */

type MyLinkedList struct {
    dummy *ListNode
    cnt   int
}

func Constructor() MyLinkedList {
    return MyLinkedList{&ListNode{}, 0}
}

func (this *MyLinkedList) Get(index int) int {
    if index < 0 || index >= this.cnt {
        return -1
    }
    cur := this.dummy.Next
    for ; index > 0; index-- {
        cur = cur.Next
    }
    return cur.Val
}

func (this *MyLinkedList) AddAtHead(val int) {
    this.AddAtIndex(0, val)
}

func (this *MyLinkedList) AddAtTail(val int) {
    this.AddAtIndex(this.cnt, val)
}

func (this *MyLinkedList) AddAtIndex(index int, val int) {
    if index > this.cnt {
        return
    }
    pre := this.dummy
    for ; index > 0; index-- {
        pre = pre.Next
    }
    pre.Next = &ListNode{val, pre.Next}
    this.cnt++
}

func (this *MyLinkedList) DeleteAtIndex(index int) {
    if index < 0 || index >= this.cnt {
        return
    }
    pre := this.dummy
    for ; index > 0; index-- {
        pre = pre.Next
    }
    t := pre.Next
    pre.Next = t.Next
    t.Next = nil
    this.cnt--
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * obj := Constructor();
 * param_1 := obj.Get(index);
 * obj.AddAtHead(val);
 * obj.AddAtTail(val);
 * obj.AddAtIndex(index,val);
 * obj.DeleteAtIndex(index);
 */

class LinkNode {
    public val: number;
    public next: LinkNode;

    constructor(val: number, next: LinkNode = null) {
        this.val = val;
        this.next = next;
    }
}

class MyLinkedList {
    public head: LinkNode;

    constructor() {
        this.head = null;
    }

    get(index: number): number {
        if (this.head == null) {
            return -1;
        }
        let cur = this.head;
        let idxCur = 0;
        while (idxCur < index) {
            if (cur.next == null) {
                return -1;
            }
            cur = cur.next;
            idxCur++;
        }
        return cur.val;
    }

    addAtHead(val: number): void {
        this.head = new LinkNode(val, this.head);
    }

    addAtTail(val: number): void {
        const newNode = new LinkNode(val);
        if (this.head == null) {
            this.head = newNode;
            return;
        }
        let cur = this.head;
        while (cur.next != null) {
            cur = cur.next;
        }
        cur.next = newNode;
    }

    addAtIndex(index: number, val: number): void {
        if (index <= 0) {
            return this.addAtHead(val);
        }
        const dummy = new LinkNode(0, this.head);
        let cur = dummy;
        let idxCur = 0;
        while (idxCur < index) {
            if (cur.next == null) {
                return;
            }
            cur = cur.next;
            idxCur++;
        }
        cur.next = new LinkNode(val, cur.next || null);
    }

    deleteAtIndex(index: number): void {
        if (index == 0) {
            this.head = (this.head || {}).next;
            return;
        }
        const dummy = new LinkNode(0, this.head);
        let cur = dummy;
        let idxCur = 0;
        while (idxCur < index) {
            if (cur.next == null) {
                return;
            }
            cur = cur.next;
            idxCur++;
        }
        cur.next = (cur.next || {}).next;
    }
}

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

#[derive(Default)]
struct MyLinkedList {
    head: Option<Box<ListNode>>,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl MyLinkedList {
    fn new() -> Self {
        Default::default()
    }

    fn get(&self, mut index: i32) -> i32 {
        if self.head.is_none() {
            return -1;
        }
        let mut cur = self.head.as_ref().unwrap();
        while index > 0 {
            match cur.next {
                None => {
                    return -1;
                }
                Some(ref next) => {
                    cur = next;
                    index -= 1;
                }
            }
        }
        cur.val
    }

    fn add_at_head(&mut self, val: i32) {
        self.head = Some(Box::new(ListNode {
            val,
            next: self.head.take(),
        }));
    }

    fn add_at_tail(&mut self, val: i32) {
        let new_node = Some(Box::new(ListNode { val, next: None }));
        if self.head.is_none() {
            self.head = new_node;
            return;
        }
        let mut cur = self.head.as_mut().unwrap();
        while let Some(ref mut next) = cur.next {
            cur = next;
        }
        cur.next = new_node;
    }

    fn add_at_index(&mut self, mut index: i32, val: i32) {
        let mut dummy = Box::new(ListNode {
            val: 0,
            next: self.head.take(),
        });
        let mut cur = &mut dummy;
        while index > 0 {
            if cur.next.is_none() {
                return;
            }
            cur = cur.next.as_mut().unwrap();
            index -= 1;
        }
        cur.next = Some(Box::new(ListNode {
            val,
            next: cur.next.take(),
        }));
        self.head = dummy.next;
    }

    fn delete_at_index(&mut self, mut index: i32) {
        let mut dummy = Box::new(ListNode {
            val: 0,
            next: self.head.take(),
        });
        let mut cur = &mut dummy;
        while index > 0 {
            if let Some(ref mut next) = cur.next {
                cur = next;
            }
            index -= 1;
        }
        cur.next = cur.next.take().and_then(|n| n.next);
        self.head = dummy.next;
    }
}

Solution 2

Thinking

Solution 1 allocates a node on every insert. The limit here is tiny, but repeated allocation dominates once the list grows large.

Preallocate two arrays for values and successor indices, and treat an increasing \(\textit{idx}\) as a node pool. Inserting claims the next slot and rewrites indices—the same linking as pointers.

\(\textit{head}\) stores the first index and \(\textit{cnt}\) the length. Asymptotics match Solution 1 without per-operation allocation.

Python3JavaC++GoTypeScript

class MyLinkedList:
    def __init__(self):
        self.e = [0] * 1010
        self.ne = [0] * 1010
        self.idx = 0
        self.head = -1
        self.cnt = 0

    def get(self, index: int) -> int:
        if index < 0 or index >= self.cnt:
            return -1
        i = self.head
        for _ in range(index):
            i = self.ne[i]
        return self.e[i]

    def addAtHead(self, val: int) -> None:
        self.e[self.idx] = val
        self.ne[self.idx] = self.head
        self.head = self.idx
        self.idx += 1
        self.cnt += 1

    def addAtTail(self, val: int) -> None:
        self.addAtIndex(self.cnt, val)

    def addAtIndex(self, index: int, val: int) -> None:
        if index > self.cnt:
            return
        if index <= 0:
            self.addAtHead(val)
            return
        i = self.head
        for _ in range(index - 1):
            i = self.ne[i]
        self.e[self.idx] = val
        self.ne[self.idx] = self.ne[i]
        self.ne[i] = self.idx
        self.idx += 1
        self.cnt += 1

    def deleteAtIndex(self, index: int) -> None:
        if index < 0 or index >= self.cnt:
            return -1
        self.cnt -= 1
        if index == 0:
            self.head = self.ne[self.head]
            return
        i = self.head
        for _ in range(index - 1):
            i = self.ne[i]
        self.ne[i] = self.ne[self.ne[i]]


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

class MyLinkedList {
    private int[] e = new int[1010];
    private int[] ne = new int[1010];
    private int head = -1;
    private int idx;
    private int cnt;

    public MyLinkedList() {
    }

    public int get(int index) {
        if (index < 0 || index >= cnt) {
            return -1;
        }
        int i = head;
        while (index-- > 0) {
            i = ne[i];
        }
        return e[i];
    }

    public void addAtHead(int val) {
        e[idx] = val;
        ne[idx] = head;
        head = idx++;
        ++cnt;
    }

    public void addAtTail(int val) {
        addAtIndex(cnt, val);
    }

    public void addAtIndex(int index, int val) {
        if (index > cnt) {
            return;
        }
        if (index <= 0) {
            addAtHead(val);
            return;
        }
        int i = head;
        while (--index > 0) {
            i = ne[i];
        }
        e[idx] = val;
        ne[idx] = ne[i];
        ne[i] = idx++;
        ++cnt;
    }

    public void deleteAtIndex(int index) {
        if (index < 0 || index >= cnt) {
            return;
        }
        --cnt;
        if (index == 0) {
            head = ne[head];
            return;
        }
        int i = head;
        while (--index > 0) {
            i = ne[i];
        }
        ne[i] = ne[ne[i]];
    }
}

/**
 * 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);
 */

class MyLinkedList {
private:
    int e[1010], ne[1010];
    int head = -1, idx = 0, cnt = 0;

public:
    MyLinkedList() {
    }

    int get(int index) {
        if (index < 0 || index >= cnt) {
            return -1;
        }
        int i = head;
        while (index--) {
            i = ne[i];
        }
        return e[i];
    }

    void addAtHead(int val) {
        e[idx] = val;
        ne[idx] = head;
        head = idx++;
        ++cnt;
    }

    void addAtTail(int val) {
        addAtIndex(cnt, val);
    }

    void addAtIndex(int index, int val) {
        if (index > cnt) {
            return;
        }
        if (index <= 0) {
            addAtHead(val);
            return;
        }
        int i = head;
        while (--index) {
            i = ne[i];
        }
        e[idx] = val;
        ne[idx] = ne[i];
        ne[i] = idx++;
        ++cnt;
    }

    void deleteAtIndex(int index) {
        if (index < 0 || index >= cnt) {
            return;
        }
        --cnt;
        if (index == 0) {
            head = ne[head];
            return;
        }
        int i = head;
        while (--index) {
            i = ne[i];
        }
        ne[i] = ne[ne[i]];
    }
};

/**
 * 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);
 */

type MyLinkedList struct {
    e    []int
    ne   []int
    idx  int
    head int
    cnt  int
}

func Constructor() MyLinkedList {
    e := make([]int, 1010)
    ne := make([]int, 1010)
    return MyLinkedList{e, ne, 0, -1, 0}
}

func (this *MyLinkedList) Get(index int) int {
    if index < 0 || index >= this.cnt {
        return -1
    }
    i := this.head
    for ; index > 0; index-- {
        i = this.ne[i]
    }
    return this.e[i]
}

func (this *MyLinkedList) AddAtHead(val int) {
    this.e[this.idx] = val
    this.ne[this.idx] = this.head
    this.head = this.idx
    this.idx++
    this.cnt++
}

func (this *MyLinkedList) AddAtTail(val int) {
    this.AddAtIndex(this.cnt, val)
}

func (this *MyLinkedList) AddAtIndex(index int, val int) {
    if index > this.cnt {
        return
    }
    if index <= 0 {
        this.AddAtHead(val)
        return
    }
    i := this.head
    for ; index > 1; index-- {
        i = this.ne[i]
    }
    this.e[this.idx] = val
    this.ne[this.idx] = this.ne[i]
    this.ne[i] = this.idx
    this.idx++
    this.cnt++
}

func (this *MyLinkedList) DeleteAtIndex(index int) {
    if index < 0 || index >= this.cnt {
        return
    }
    this.cnt--
    if index == 0 {
        this.head = this.ne[this.head]
        return
    }
    i := this.head
    for ; index > 1; index-- {
        i = this.ne[i]
    }
    this.ne[i] = this.ne[this.ne[i]]
}

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * obj := Constructor();
 * param_1 := obj.Get(index);
 * obj.AddAtHead(val);
 * obj.AddAtTail(val);
 * obj.AddAtIndex(index,val);
 * obj.DeleteAtIndex(index);
 */

class MyLinkedList {
    e: Array<number>;
    ne: Array<number>;
    idx: number;
    head: number;
    cnt: number;

    constructor() {
        this.e = new Array(1010).fill(0);
        this.ne = new Array(1010).fill(0);
        this.head = -1;
        this.idx = 0;
        this.cnt = 0;
    }

    get(index: number): number {
        if (index < 0 || index >= this.cnt) {
            return -1;
        }
        let i = this.head;
        while (index--) {
            i = this.ne[i];
        }
        return this.e[i];
    }

    addAtHead(val: number): void {
        this.e[this.idx] = val;
        this.ne[this.idx] = this.head;
        this.head = this.idx++;
        this.cnt++;
    }

    addAtTail(val: number): void {
        this.addAtIndex(this.cnt, val);
    }

    addAtIndex(index: number, val: number): void {
        if (index > this.cnt) {
            return;
        }
        if (index <= 0) {
            this.addAtHead(val);
            return;
        }
        let i = this.head;
        while (--index) {
            i = this.ne[i];
        }
        this.e[this.idx] = val;
        this.ne[this.idx] = this.ne[i];
        this.ne[i] = this.idx++;
        this.cnt++;
    }

    deleteAtIndex(index: number): void {
        if (index < 0 || index >= this.cnt) {
            return;
        }
        this.cnt--;
        if (index == 0) {
            this.head = this.ne[this.head];
            return;
        }
        let i = this.head;
        while (--index) {
            i = this.ne[i];
        }
        this.ne[i] = this.ne[this.ne[i]];
    }
}

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

707. Design Linked List

Description

Solutions

Solution 1

Solution 2

Comments