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40--reverse-linked-list.java
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40--reverse-linked-list.java
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/**
* 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) return head;
ListNode prev = null;
ListNode curr = head;
while (curr != null) {
ListNode tmp = curr.next;
curr.next = prev;
prev = curr;
curr = tmp;
}
return prev;
}
}
class Solution {
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null)
return head;
return reverse(head, null); // head, newHead
}
private ListNode reverse(ListNode head, ListNode newHead) {
if (head == null) {
return newHead;
}
ListNode next = head.next;
head.next = newHead;
return reverse(next, head);
}
} // TC: O(n), SC: O(n)
class Solution {
ListNode newHead = null;
public ListNode reverseList(ListNode head) {
if (head == null || head.next == null)
return head;
reverse(head);
return newHead;
}
private ListNode reverse(ListNode head) {
if (head.next == null) {
newHead = head;
return head;
}
ListNode tmp = reverse(head.next);
tmp.next = head;
head.next = null;
return head;
}
} // TC: O(n), SC: O(n)