1290. Convert Binary Number in a Linked List to Integer - LeetCode Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def getDecimalValue(self, head: ListNode) -> int:
        ans=0
        i = 0
        root = head
        while root:
            i+=1
            root = root.next
        i-=1
        while head:
            ans+= (head.val)*(2**i)
            head = head.next
            i-=1
        return ans