class Node:
    def __init__(self, data):
        self.data = data
        self.prev = None
        self.next = None

class DoublyLinkedList:
    def __init__(self):
        self.head = None

    def append(self, data):
        new_node = Node(data)
        if not self.head:
            self.head = new_node
            return
        current = self.head
        while current.next:
            current = current.next
        current.next = new_node
        new_node.prev = current

    def insert(self, index, data):
        new_node = Node(data)
        if index == 0:
            new_node.next = self.head
            if self.head:
                self.head.prev = new_node
            self.head = new_node
            return
        current = self.head
        for _ in range(index - 1):
            if not current:
                raise IndexError("Index out of bounds")
            current = current.next
        if not current:
            raise IndexError("Index out of bounds")
        new_node.next = current.next
        new_node.prev = current
        if current.next:
            current.next.prev = new_node
        current.next = new_node

    def delete(self, data):
        current = self.head
        while current:
            if current.data == data:
                if current.prev:
                    current.prev.next = current.next
                else:
                    self.head = current.next
                if current.next:
                    current.next.prev = current.prev
                return
            current = current.next
        raise ValueError(f"{data} not found in list")

    def reverse(self):
        current = self.head
        prev_node = None
        while current:
            next_node = current.next
            current.next = prev_node
            current.prev = next_node
            prev_node = current
            current = next_node
        self.head = prev_node

    def find(self, data):
        current = self.head
        index = 0
        while current:
            if current.data == data:
                return index
            current = current.next
            index += 1
        return -1
    
    def to_list(self):
        result = []
        current = self.head
        while current:
            result.append(current.data)
            current = current.next
        return result

    def display_forward(self):
        current = self.head
        while current:
            print(current.data, end=" ⇄ ")
            current = current.next
        print("None")

    def display_backward(self):
        current = self.head
        if not current:
            print("None")
            return
        while current.next:
            current = current.next
        while current:
            print(current.data, end=" ⇄ ")
            current = current.prev
        print("None")
        
dll = DoublyLinkedList()
dll.append(10)
dll.append(20)
dll.append(30)
dll.insert(1, 15)       # Insert 15 at index 1
dll.delete(20)          # Remove node with value 20
dll.reverse()           # Reverse the list
print("Index of 15:", dll.find(15))
dll.append(50)
print(dll.to_list())  # Output: [30, 15, 10, 50]        


dll.display_forward()   # Output: 30 ⇄ 15 ⇄ 10 ⇄ None
dll.display_backward()  # Output: 10 ⇄ 15 ⇄ 30 ⇄ None