1290. Convert Binary Number in a Linked List to Integer

Given head which is a reference node to a singly-linked list. The value of each node in the linked list is either 0 or 1. The linked list holds the binary representation of a number.

Return the decimal value of the number in the linked list.

Example 1:

Input: head = [1,0,1]
Output: 5
Explanation: (101) in base 2 = (5) in base 10

Example 2:

Input: head = [0]
Output: 0

Example 3:

Input: head = [1]
Output: 1

Example 4:

Input: head = [1,0,0,1,0,0,1,1,1,0,0,0,0,0,0]
Output: 18880

Example 5:

Input: head = [0,0]
Output: 0

Constraints:

• The Linked List is not empty.
• Number of nodes will not exceed 30.
• Each node’s value is either 0 or 1.

Solution

class Solution:
    def getDecimalValue(self, head: ListNode) -> int:
        
        result = 0
        
        while head:
            result = (result << 1) | head.val
            head = head.next
        
        return result

In this solution we have an integer, result, in which we store our current result. As we loop through each node in the linked list, we take the bit value at each node and update our result accordingly.

The algorithm goes like this:

• For each node in the list
  • Left Shift result by 1 bit to make space for the new bit value
  • Set result equal to result OR new_bit
  • Update our current node and move to the next one
• When we’re finished, return our result.

Notes

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