Description
Reverse a singly linked list.
Example:
Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL
Follow up:
A linked list can be reversed either iteratively or recursively. Could you implement both?
Solution1 - 使用栈
class ListNode {
int val;
ListNode next;
ListNode(int x) {
val = x;
next = null;
}
}
class Solution {
public static ListNode reverseList(ListNode head) {
if (head == null || head.next == null)
return head;
ListNode left = head;
ListNode right = left.next;
left.next = null;
while (right != null) {
ListNode temp = right.next;
right.next = left;
left = right;
right = temp;
}
return left;
}
}
public class Test {
public static void main(String[] args) {
ListNode n1 = new ListNode(1);
ListNode n2 = new ListNode(2);
ListNode n3 = new ListNode(3);
ListNode n4 = new ListNode(4);
n1.next = n2;
n2.next = n3;
n3.next = n4;
Solution.reverseList(n1);
}
}
Solution2
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
class Solution {
public ListNode reverseList(ListNode head) {
ListNode left = null;
ListNode right = head;
while(right != null){
ListNode tmp = right.next;
right.next = left;
left = right;
right = tmp;
}
return left;
}
}
Solution3 - 递归
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null) return head;
ListNode p = reverseList(head.next);
head.next.next = head;
head.next = null;
return p;
}