LeetCode 206. Reverse linked list
using virtual head node
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode vh;
vh.next = NULL;
ListNode *p = head;
ListNode *q;
while (p) {
q = p->next;
p->next = vh.next;
vh.next = p;
p = q;
}
return vh.next;
}
};
Do not use virtual head nodes
class Solution {
public:
ListNode* reverseList(ListNode* head) {
ListNode *ret = NULL, *p = head, *q;
while (p) {
q = p->next;
p->next = ret;
ret = p;
p = q;
}
return ret;
}
};
recursive writing
class Solution {
public:
ListNode* reverseList(ListNode* head) {
if (head == NULL || head->next == NULL) {
return head;
}
ListNode *next = reverseList(head->next);
head->next->next = head;
head->next = NULL;
return next;
}
};