2816. Double a Number Represented as a Linked List

T: O(n)

S: O(n)

```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; }
 * }
 */
import java.math.*;
class Solution {
    public ListNode doubleIt(ListNode head) {
        StringBuilder sb = new StringBuilder();
        ListNode dummy = head;
        while (dummy != null) {
            sb.append(dummy.val);
            dummy = dummy.next;
        }
        //System.out.println(sb.toString());
        // BigDecimal out = new BigDecimal(sb.toString());
        // BigDecimal mOut = out.multiply(new BigDecimal("2"));
        // String o = String.valueOf(mOut);
        
        int carry = 0;
        char[] c = sb.toString().toCharArray();

        int num = (c[c.length-1] - '0')*2;
        carry = num/10;
        num = num%10;

        ListNode pre = new ListNode(num);
        if (c.length == 1 && carry == 0) {
            return pre;
        }

        ListNode cur = null;
        for (int i = c.length-2; i >= 0; i--) {
            num = (c[i] - '0')*2 + carry;

            carry = num/10;
            num = num%10;

            cur = new ListNode(num);
            cur.next = pre;
            pre = cur;
        }

        if (carry != 0) {
            cur = new ListNode(carry);
            cur.next = pre;
        }
        return cur;
    }
}
```

Smarter - use recursion

T :O(n)

S: O(1)

```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 doubleIt(ListNode head) {
        int t = multiply(head);
        ListNode cur = head;
        if (t != 0) {
            cur = new ListNode(t);
            cur.next = head;
        }
        return cur;
    }
    private int multiply(ListNode node) {
        if (node == null) {
            return 0;
        }
        int t = node.val*2 + multiply(node.next);
        node.val = t%10;
        return t/10; // carry
    }
}
```