单链表

class MyLinkedList:
    def __init__(self, head=None, size=0):
        self.head = head
        self.size = size

    def get(self, index):
        if index >= self.size or not self.head:
            return -1
        cur = self.head
        while index:
            cur = cur.next
            index -= 1
        return cur.val

    def add_at_head(self, val):
        self.head = ListNode(val, self.head)
        self.size += 1

    def add_at_tail(self, val):
        if not self.head:
            self.head = ListNode(val)
        else:
            cur = self.head
            while cur.next:
                cur = cur.next
            cur.next = ListNode(val)
        self.size += 1

    def add_at_index(self, val, index):
        if not index > self.size:
            if index <= 0:
                self.add_at_head(val)
            else:
                cur = self.head
                while index - 1:
                    cur = cur.next
                    index -= 1
                cur.next = ListNode(val, cur.next)
                self.size += 1

    def delete_at_index(self, index):
        if index < self.size:
            if not index:
                self.head = self.head.next
            else:
                cur = self.head
                while index - 1:
                    cur = cur.next
                    index -= 1
                cur.next = cur.next.next
            self.size -= 1