tow pointers: suitable for arrays with positive numbers
pre-sum: ① array / HashMap; ② suitable for arrays with both positive and negative numbers; ③ cautions to overflow, especially with multiplication

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2

public static ListNode delete(ListNode head, int value)
node.next = node.next.next;

Because the head may be deleted, so return a listnode instead of void.
PS: revise the Nth node, operate with the (N-1)th node

[go through one time] find the (n+1)th node from end
[Method] two pointers

• P1 go n steps
• P2 go from dummy

• if there is a loop in the list
  two points, one go fast (2 steps a time), one go slow; if they meet
• locate the entry when n nodes in the loop
  if there are n nodes, P1 go n steps in advance, P2 then follow, they meet at the entry
• how to count
  find any loop node and move along until go back

[method1] find the length of L1 and L2
P1 go advance (L1-L2) steps
[method2] make a loop by connecting L1’s head with common’s tail node

[method1] Stack [O(n)space]
[method2] recursion
[method3] know the ith next previous node

reverse L1 and L2 and add in digits and remember the carry-bit
PS: the result’s length might be longer than the origial

• reverse L1, L2
• reverse add(L1,L2)

{
ListNode next;
ListNode prev;
}

[Rotation] waiting to study

waiting to study
new node: max, min, in between
using prev, next

• in between: prev < n < next
• min: after maxList
• max: after maxList
  special case: only 1 node; null list

• empty lists
• lists with only node
• head node
• tail node