369. Plus One Linked List 🔒

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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. 

Solutions

Solution 1: Linked List Traversal

We first set a dummy head node $\textit{dummy}$, initially with a value of $0$, and the successor node of $\textit{dummy}$ is the linked list $\textit{head}$.

Next, we traverse the linked list starting from the dummy head node, find the last node that is not $9$, increment its value by $1$, and set the values of all nodes after this node to $0$.

Finally, we check if the value of the dummy head node is $1$. If it is $1$, we return $\textit{dummy}$; otherwise, we return the successor node of $\textit{dummy}$.

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

Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def plusOne(self, head: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode(0, head)
        target = dummy
        while head:
            if head.val != 9:
                target = head
            head = head.next
        target.val += 1
        target = target.next
        while target:
            target.val = 0
            target = target.next
        return dummy if dummy.val else dummy.next

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode plusOne(ListNode head) {
        ListNode dummy = new ListNode(0, head);
        ListNode target = dummy;
        while (head != null) {
            if (head.val != 9) {
                target = head;
            }
            head = head.next;
        }
        ++target.val;
        target = target.next;
        while (target != null) {
            target.val = 0;
            target = target.next;
        }
        return dummy.val == 1 ? dummy : dummy.next;
    }
}

C++

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* plusOne(ListNode* head) {
        ListNode* dummy = new ListNode(0, head);
        ListNode* target = dummy;
        for (; head; head = head->next) {
            if (head->val != 9) {
                target = head;
            }
        }
        target->val++;
        for (target = target->next; target; target = target->next) {
            target->val = 0;
        }
        return dummy->val ? dummy : dummy->next;
    }
};

Go

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
func plusOne(head *ListNode) *ListNode {
	dummy := &ListNode{0, head}
	target := dummy
	for head != nil {
		if head.Val != 9 {
			target = head
		}
		head = head.Next
	}
	target.Val++
	for target = target.Next; target != nil; target = target.Next {
		target.Val = 0
	}
	if dummy.Val == 1 {
		return dummy
	}
	return dummy.Next
}

TypeScript

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function plusOne(head: ListNode | null): ListNode | null {
    const dummy = new ListNode(0, head);
    let target = dummy;
    while (head) {
        if (head.val !== 9) {
            target = head;
        }
        head = head.next;
    }
    target.val++;
    for (target = target.next; target; target = target.next) {
        target.val = 0;
    }
    return dummy.val ? dummy : dummy.next;
}