My Java solution with o(n) time complexity and o(1) space complexity:

public static SinglyLinkedListNode reverse(SinglyLinkedListNode llist) { if(llist == null) return null; if(llist.next == null) return llist;

    SinglyLinkedListNode curr = llist;
    SinglyLinkedListNode prev = null;
    SinglyLinkedListNode temp = null; 

    while(curr != null){
        temp = curr.next;
        //point curr node to prev
        curr.next = prev;
        //set prev = curr node
        prev = curr;
        curr = temp;
    }

    return prev;
}