206. Reverse Linked List

1. Question

Given the head of a singly linked list, reverse the list, and return the reversed list.

2. Examples

Example 1:

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

3. Constraints

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/reverse-linked-list 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

迭代，在原来的列表上改变next的指向。

/**
 * 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) {
    ListNode prev = null;
    ListNode curr = head;
    while(curr != null) {
      ListNode next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }
    return prev;
  }
}