2816. Double a Number Represented as a Linked List

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# 題目敘述

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.

// 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; }

}

# 解題思路

# Solution

import java.util.Stack;

class Solution {

    public ListNode doubleIt(ListNode head) {

        Stack<Integer> stack = new Stack<>();

        int val = 0;

        while (head != null) {

            stack.push(head.val);

            head = head.next;

        }

        ListNode ans = null;

        while (!stack.isEmpty() || val != 0) {

            ans = new ListNode(0, ans);

            if (!stack.isEmpty()) {

                val += stack.pop() * 2;

            }

            ans.val = val % 10;

            val /= 10;

        }

        return ans;

    }

}