🧠 Solving LeetCode Until I Become Top 1% — Day `43`

🔹 Problem: 1290. Convert Binary Number in a Linked List to Integer

Difficulty: #Easy
Tags: #LinkedList, #Math, #BitManipulation

📝 Problem Summary

You’re given the head of a singly linked list where each node contains either a 0 or a 1, and the entire linked list represents a binary number (most significant bit comes first).

Your task is to convert that binary number to its decimal (base 10) form and return it as an integer.

🧠 My Thought Process

Brute Force Idea:
- Traverse the linked list and append each bit to a string.
- After the traversal, use Python's built-in int(binary_string, 2) to convert it.
Optimized Strategy:
- Although string-building is simple, we could optimize space by keeping a running integer.
- For every bit b, left-shift the current result (res = res * 2 + b).

But for now, the string approach is fast and clear, especially for small constraints.

Algorithm Used: Basic linked list traversal and binary string conversion.

⚙️ Code Implementation (Python)

class Solution:
    def getDecimalValue(self, head: Optional[ListNode]) -> int:
        s = ''
        while head:
            s += str(head.val)
            head = head.next
        return int(s, 2)

⏱️ Time & Space Complexity

Time: O(n) — one traversal through the list
Space: O(n) — string s stores n bits

⚠️ This could be optimized to O(1) space by using integer shifting instead of a string.

🧩 Key Takeaways

✅ I practiced converting binary to decimal using both string manipulation and bit math.
💡 It's okay to start with readable solutions first, then optimize if necessary.
💭 Bit manipulation and linked list problems often come together — be prepared for that combo.