The first step to solve this problem is to understand what is singly linked list. A singly linked list is a collection of nodes where each node points to the next node in the list and the last node points to null. Reversing a singly linked list means changing the direction of each pointer from pointing to the next node to pointing to the previous node.
The function to reverse a singly linked list in-place can be implemented using an iterative approach or a recursive approach. Here is an example of the iterative approach:
public ListNode reverseList(ListNode head) {
ListNode prev = null;
ListNode current = head;
ListNode next = null;
while(current != null) {
next = current.next;
current.next = prev;
prev = current;
current = next;
}
return prev;
}
The above function takes the head of the linked list as input and returns the head of the reversed linked list as output. It uses three pointers, prev, current, and next, to reverse the linked list in-place. The pointer prev initially points to null, the pointer current initially points to the head of the linked list, and the pointer next initially points to null. The while loop traverses the linked list until the current pointer becomes null. In each iteration, the pointer next is used to temporarily store the reference to the next node, the pointer current.next is changed to point to the previous node (which is pointed by the pointer prev), the pointer prev is updated to point to the current node, and the pointer current is updated to point to the next node.
Here is an example of the recursive approach:
public ListNode reverseList(ListNode head) {
if(head == null || head.next == null) {
return head;
}
ListNode newHead = reverseList(head.next);
head.next.next = head;
head.next = null;
return newHead;
}
The above function also takes the head of the linked list as input and returns the head of the reversed linked list as output. It uses a recursive approach to reverse the linked list in-place. The base case of the recursion is when the head of the linked list is null or the next node is null. In the recursive case, the function recursively calls itself with the next node as input and uses two pointers, head and newHead, to reverse the linked list in-place. The pointer newHead initially points to the last node of the original linked list after the recursion reaches the base case. The pointer head.next.next is used to change the direction of each pointer from pointing to the next node to pointing to the previous node, and the pointer head.next is used to set the next pointer of the original last node to null.