LeetCode-in-Java

2816. Double a Number Represented as a Linked List

Medium

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.

Solution

import com_github_leetcode.ListNode;

/*
 * 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; }
 * }
 */
public class Solution {
    public ListNode doubleIt(ListNode head) {
        ListNode temp1 = revList(head);
        ListNode list1 = null;
        ListNode current = list1;
        int carry = 0;
        while (temp1 != null) {
            int val = temp1.val * 2;
            if (list1 == null) {
                list1 = new ListNode(val % 10 + carry);
                current = list1;
            } else {
                current.next = new ListNode(val % 10 + carry);
                current = current.next;
            }
            carry = val / 10;
            temp1 = temp1.next;
        }
        if (carry == 1) {
            current.next = new ListNode(carry);
        }
        return revList(list1);
    }

    private ListNode revList(ListNode head) {
        ListNode prev = null;
        ListNode nxt = null;
        ListNode current = head;
        while (current != null) {
            nxt = current.next;
            current.next = prev;
            prev = current;
            current = nxt;
        }
        return prev;
    }
}

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