206. reverse linked list

Train of thought Change pointer direction

Main idea : Traverse all nodes , Change the direction of the node

1. The initial state : last Point to null, head Point to the head node

1. Define a nextHead node , Used to store head.next The node of , Otherwise, once the direction changes , head.next The node is lost

1. Then change head Direction of edges of , head.next = last

1. last and head Move forward one step

1. Then cycle the above process

class Solution {
    
        public ListNode reverseList(ListNode head) {
    

            ListNode last = null;

            while(head != null) {
    
                //  preservation head.next ,  as a result of   When the direction of the edge changes , head.next  Nodes of will be lost ,  So save it in advance 
                ListNode nextHead = head.next;

                //  Change the direction of the edge 
                head.next = last;

                // last  and  head  Move forward one step 
                last = head;
                head = nextHead;
            }
            return last;
        }
    }