力扣23题

public ListNode mergeTwoLists(ListNode a, ListNode b) {
    if (a == null || b == null) {
        return a != null ? a : b;
    }
    ListNode head = new ListNode(0);
    ListNode tail = head, aPtr = a, bPtr = b;
    while (aPtr != null && bPtr != null) {
        if (aPtr.val < bPtr.val) {
            tail.next = aPtr;
            aPtr = aPtr.next;
        } else {
            tail.next = bPtr;
            bPtr = bPtr.next;
        }
        tail = tail.next;
    }
    tail.next = (aPtr != null ? aPtr : bPtr);
    return head.next;
}