Guidelines

When it comes to linked list practice makes perfect, but there are few tricks you should remember when dealing with them:

// Reverse linked list
while(next != null){
    curr = next;
    next = next.next;
    curr.next = prev;
    prev = curr;
}

// use dummy pointer to make the implemenation easier
ListNode dummy = new ListNode(-1, null)
ListNode iter = dummy;

// fast, slow pointer
ListNode slow = head, fast = head;
while(fast != null && fast.next != null){
    slow = slow.next;
    fast = fast.next.next;
}
return slow;

// skipping an element by checking and relinking ahead
if(curr.next.val == val){
    curr.next = curr.next.next;
}

while (l1 != null || l2 != null || carry != 0) // for adding two lists

// reversing a sublist in place bubble out the first element to end
for(int i = 0; i < right - left; ++i){
    ListNode next = iter.next;
    iter.next = next.next;
    next.next = start.next;
    start.next = next;
}