369. Plus One Linked List - Explanation

Problem Link

Description

Given a non-negative integer represented as a linked list of digits, plus one to the integer.

The digits are stored such that the most significant digit is at the head of the list.

Example 1:

Input: head = [1,2,3]

Output: [1,2,4]

Example 2:

Input: head = [0]

Output: [1]

Constraints:

• The number of nodes in the linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• The number represented by the linked list does not contain leading zeros except for the zero itself.

Company Tags

Google3 Amazon3

1. Sentinel Head + Textbook Addition

class Solution:
    def plusOne(self, head: ListNode) -> ListNode:
        # sentinel head
        sentinel = ListNode(0)
        sentinel.next = head
        not_nine = sentinel

        # find the rightmost not-nine digit
        while head:
            if head.val != 9:
                not_nine = head
            head = head.next

        # increase this rightmost not-nine digit by 1
        not_nine.val += 1
        not_nine = not_nine.next

        # set all the following nines to zeros
        while not_nine:
            not_nine.val = 0
            not_nine = not_nine.next

        return sentinel if sentinel.val else sentinel.next

Time & Space Complexity

• Time complexity: $O(N)$
• Space complexity: $O(1)$

Where $N$ is the length of the input list