Today I have done leetcode's May LeetCoding Challenge with cpp.

May LeetCoding Challenge 7

Description

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.

Solution

class Solution {
  int helper(ListNode *head) {
    if(!head) return 0;
    head->val *= 2;
    if(helper(head->next)) {
      head->val += 1;
    }
    if(head->val > 9) {
      head->val -= 10;
      return 1;
    }
    return 0;
  }
public:
  ListNode* doubleIt(ListNode* head) {
    if(helper(head)) {
      return new ListNode(1, head);
    }
    return head;
  }
};