[LeetCode] 206. Reverse Linked List

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */ 
public class Solution {
    /**
     * @param head: The head of linked list.
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        // write your code here
        ListNode previous = null;
        while(head != null){
            //Remember: assigne as circle
            ListNode temp = head.next; 
            head.next = previous; 
            previous = head; 
            head = temp;
        }
        return previous;
    }
}

最佳方法去记住list修改的顺序：这是一个循环，从temp开始，上一行等号右边就是下一行等号左边的。一直到循环结束。这个方法考虑了empty list以及只有一个node的list

下边也是一个思路，只是更容易理解而已。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode prev = null;
        ListNode next = null;
        while (head != null) {
            next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}