Merge k Sorted Lists Test

The solution approach is to take a reference to a vector of ListNode pointers lists as input and returns a pointer to the head of the merged linked list. The function first checks the size of the lists vector. If it is empty, it returns nullptr since there are no linked lists to merge. If there is only one linked list in the vector, it directly returns that list since it is already sorted. The function creates two new vectors of ListNode pointers, lists1 and lists2. lists1 contains the first half of the linked lists from the lists vector, and lists2 contains the second half. This splitting is done by copying the appropriate elements from lists using vector slicing. The function recursively calls mergeKLists on lists1 and lists2 to merge the two halves of the linked lists and obtain two merged sorted linked lists l1 and l2. If l1 is nullptr, it means there were no linked lists in lists1 or the merged result of lists1 is empty. In this case, the function directly returns l2 since it represents the merged result. The variable ret is initialized to l1, which will be used to store the head of the merged result. The function enters a while loop that runs as long as l2 is not nullptr. This loop is responsible for merging the two sorted linked lists l1 and l2. Inside the loop, there is an if statement that checks if the value of the current node in l1 is greater than the value of the current node in l2. If it is, the values of the two nodes are swapped so that l1 always contains the smaller value. There is another while loop nested inside the outer while loop. This nested loop finds the correct position to insert the current node of l2 into l1. It iterates as long as l1->next is not nullptr and the value of l1->next is less than the value of the current node in l2. This loop moves l1 forward until it finds the correct position. Once the correct position is found, the next pointer of the current node in l2 is updated to point to l1->next, and the next pointer of l1 is updated to point to the current node in l2. This effectively inserts the current node of l2 into the correct position in the merged list. After the insertion, the next node of l2 is stored in a temporary variable tmp2 and l2 is updated to point to the next node. This step is performed to continue the merging process for the remaining nodes in l2. Once the while loop finishes, the merged result is stored in ret, which contains the head of the merged list. Finally, the function returns ret, which represents the head of the merged linked list.