2807. Insert Greatest Common Divisors in Linked List

 C++

class Solution {
public:
    ListNode* insertGreatestCommonDivisors(ListNode* head) {
        ListNode* pre = head;
        for (ListNode* cur = head->next; cur; cur = cur->next) {
            int x = gcd(pre->val, cur->val);
            pre->next = new ListNode(x, cur);
            pre = cur;
        }
        return head;
    }
};

JAVA

class Solution {
    public ListNode insertGreatestCommonDivisors(ListNode head) {
        for (ListNode pre = head, cur = head.next; cur != null; cur = cur.next) {
            int x = gcd(pre.val, cur.val);
            pre.next = new ListNode(x, cur);
            pre = cur;
        }
        return head;
    }

    private int gcd(int a, int b) {
        if (b == 0) {
            return a;
        }
        return gcd(b, a % b);
    }
}