707. Design Linked List

基本操作，注意函数的复用，addAtHead和addAtTail可以用addAtIndex来写

class Node(object):

    def __init__(self, val):
        self.val = val
        self.next = None


class MyLinkedList(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.head = None
        self.size = 0

    def get(self, index):
        """
        Get the value of the index-th node in the linked list. If the index is invalid, return -1.
        :type index: int
        :rtype: int
        """
        if index < 0 or index >= self.size:
            return -1

        if self.head is None:
            return -1

        curr = self.head
        for i in range(index):
            curr = curr.next
        return curr.val

    def addAtHead(self, 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.
        :type val: int
        :rtype: void
        """
        self.addAtIndex(0, val)

    def addAtTail(self, val):
        """
        Append a node of value val to the last element of the linked list.
        :type val: int
        :rtype: void
        """
        self.addAtIndex(self.size, val)

    def addAtIndex(self, index, val):
        """
        Add a node of value val before the index-th node in the linked list.
        If index equals to the length of linked list, the node will be appended to the end of linked list.
        If index is greater than the length, the node will not be inserted.
        :type index: int
        :type val: int
        :rtype: void
        """
        if index < 0 or index > self.size:
            return

        node = Node(val)

        if index == 0:
            node.next = self.head
            self.head = node
        else:
            curr = self.head
            for i in range(index - 1):
                curr = curr.next
            node.next = curr.next
            curr.next = node

        self.size += 1

    def deleteAtIndex(self, index):
        """
        Delete the index-th node in the linked list, if the index is valid.
        :type index: int
        :rtype: void
        """
        if index < 0 or index >= self.size:
            return

        curr = self.head
        if index == 0:
            self.head = curr.next
        else:
            for i in range(index - 1):
                curr = curr.next
            curr.next = curr.next.next

        self.size -= 1