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Convert Binary Number in a Linked List to Integer - Solution & Explanation

EasyLinked ListMath17 min readAsked at: Amazon, Microsoft, Meta +7
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Problem Statement

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.

The most significant bit is at the head of 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

 

Constraints:

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

Approach Overview

Problem Overview: A singly linked list stores a binary number where each node contains either 0 or 1. The head represents the most significant bit. Traverse the list and convert this binary sequence into its decimal integer value.

Approach 1: Bit Manipulation (O(n) time, O(1) space)

This approach treats the traversal like constructing a binary number bit by bit. Start with result = 0. For every node, shift the current value left by one (result << 1) and add the node’s bit using | node.val or simple addition. Each iteration effectively multiplies the current value by two and appends the new bit. The algorithm performs a single pass through the linked list, making it the most direct and efficient solution. Time complexity is O(n) and space complexity is O(1).

Approach 2: Reverse Iteration and Power of Two (O(n) time, O(n) space)

This method first collects all node values into an array while iterating through the list. After that, iterate the array from right to left and compute the decimal value using powers of two. For each bit at index i, add bit * 2^position to the result. The logic mirrors standard binary-to-decimal conversion using math. While still linear time, this approach uses extra memory to store the bits before computing the final value. Time complexity is O(n) and space complexity is O(n).

Recommended for interviews: The Bit Manipulation approach is the expected solution. It processes the list in one pass with constant space and clearly demonstrates understanding of binary operations. The power-of-two approach works but adds unnecessary memory overhead. Interviewers typically want to see how you apply bit manipulation while traversing a linked list.

Approach 1: Bit Manipulation

This approach involves traversing the linked list and building the decimal value using bit manipulation. As the linked list is traversed, the current result is shifted left by one bit (equivalent to multiplying by 2), and the current node's value is added using the OR operation.

The function getDecimalValue traverses the linked list, updating the result by left-shifting it by one bit and then OR-ing it with the current node's value. This operation effectively constructs the binary number bit by bit.

Code

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Complexity

Time Complexity: O(n), where n is the number of nodes in the linked list.
Space Complexity: O(1), as we only use a constant amount of additional space.

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Approach 2: Reverse Iteration and Power of Two

This approach involves traversing the linked list twice. First, to count the nodes (and hence determine the power of two to start with) and second to compute the decimal value by iterating through the list again, multiplying each binary digit by the appropriate power of two.

In this implementation, we traverse the list to calculate the length, which aids in determining the highest power of two needed. A second pass through the list allows for calculation of the decimal value, multiplying each node value by 2 raised to the correct power.

Code

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Complexity

Time Complexity: O(n), where n is the number of nodes, but involves two passes.
Space Complexity: O(1), since it uses constant space plus a few variables.

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Approach 3: Traverse the Linked List

We use a variable ans to record the current decimal value, with an initial value of 0.

Traverse the linked list. For each node, left-shift ans by one bit, then perform a bitwise OR with the current node's value. After traversal, ans is the decimal value.

The time complexity is O(n), where n is the length of the linked list. The space complexity is O(1).

Code

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Complexity Comparison

ApproachComplexity
Bit Manipulation

Time Complexity: O(n), where n is the number of nodes in the linked list.
Space Complexity: O(1), as we only use a constant amount of additional space.

Reverse Iteration and Power of Two

Time Complexity: O(n), where n is the number of nodes, but involves two passes.
Space Complexity: O(1), since it uses constant space plus a few variables.

Traverse the Linked List—

Detailed Complexity Analysis

ApproachTimeSpaceWhen to Use
Bit ManipulationO(n)O(1)Best general solution. Single pass through the linked list with constant memory.
Reverse Iteration and Power of TwoO(n)O(n)Useful for learning binary-to-decimal conversion or when storing nodes in an array for later processing.

Video Solution

Convert Binary Number in a Linked List to Integer | LeetCode 1290 | C++, Java, Python • Knowledge Center • 14,820 views views

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Frequently Asked Questions

Is Convert Binary Number in a Linked List to Integer easy or hard?
Convert Binary Number in a Linked List to Integer is classified as an Easy problem with an acceptance rate above 80%. The challenge focuses on recognizing the binary accumulation pattern while traversing a linked list.
Convert Binary Number in a Linked List to Integer Python/Java solution
In Python or Java, iterate through the linked list and update an integer variable using result = (result << 1) + node.val. The same logic works across C++, JavaScript, and C#. This approach maintains O(n) time and O(1) space.
How to solve Convert Binary Number in a Linked List to Integer in O(n)?
Traverse the linked list from head to tail while maintaining an integer accumulator. At each node compute result = (result << 1) + node.val. The left shift multiplies the current number by two and appends the next binary digit, producing the decimal value in a single pass.
What is the best approach for Convert Binary Number in a Linked List to Integer?
The best approach uses bit manipulation while traversing the linked list. Start with result = 0, then for each node shift the current result left by one and add the node value. This builds the binary number incrementally in O(n) time and O(1) space.
Is Convert Binary Number in a Linked List to Integer asked at Google/Amazon/Meta?
Binary conversion and linked list traversal problems appear frequently in technical interviews at companies like Amazon, Google, and Meta. This specific problem is commonly used to test understanding of bit manipulation and basic linked list iteration.
What data structure is used in Convert Binary Number in a Linked List to Integer?
The core data structure is a singly linked list where each node stores a binary digit (0 or 1). The algorithm processes nodes sequentially and may optionally use bit manipulation operations to construct the final integer value.
What is the time complexity of Convert Binary Number in a Linked List to Integer?
The optimal solution runs in O(n) time because each node in the linked list is visited exactly once. Space complexity is O(1) when using the bit manipulation approach since no extra data structures are required.

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