2816.DoubleIt.cs

// 2816. Double a Number Represented as a Linked List
// You are given the head of a non-empty linked list representing a non-negative integer without leading zeroes.

// Return the head of the linked list after doubling it.

// Example 1:
// Input: head = [1,8,9]
// Output: [3,7,8]
// Explanation: The figure above corresponds to the given linked list which represents the number 189. Hence, the returned linked list represents the number 189 * 2 = 378.
// Example 2:
// Input: head = [9,9,9]
// Output: [1,9,9,8]
// Explanation: The figure above corresponds to the given linked list which represents the number 999. Hence, the returned linked list reprersents the number 999 * 2 = 1998. 

// Constraints:
// The number of nodes in the list is in the range [1, 104]
// 0 <= Node.val <= 9
// The input is generated such that the list represents a number that does not have leading zeros, except the number 0 itself.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode DoubleIt(ListNode head) {
        ListNode first = new ListNode(0, head), prev = first;
        while(head!=null) {
            int d = head.val * 2;
            if (d>=10) prev.val++;
            head.val = d%10;
            prev = head;
            head = head.next;
        }
        return first.val > 0 ? first : first.next;
    }
}