1290. Convert Binary Number in a Linked List to Integer

Python

First try, O(2N)

class Solution:
    def getDecimalValue(self, head: ListNode) -> int:
        length = 0
        node = head
        while node:
            length += 1
            node = node.next

        result = 0
        exp = length - 1

        node = head
        while node:
            result += node.val * (2 ** exp)
            exp = exp - 1
            node = node.next
        return result

Second try, optimize version, O(N + logN)

class Solution:
    def getDecimalValue(self, head: ListNode) -> int:
        length = 0
        node = head
        while node:
            length += 1
            node = node.next

        result = 0
        exp = 2 ** (length - 1)

        node = head
        while node:
            if node.val:
                result += node.val * exp
            exp = exp // 2
            node = node.next
        return result