[数据结构与算法]1. Reverse Linked List

?struct ListNode {
?????????int? val;
?????????ListNode *next;
?????????ListNode() : val(0), next(nullptr) {}
?????????ListNode(int x) : val(x), next(nullptr) {}
?????????ListNode(int x, ListNode *next) : val(x), next(next) {}
};

?// iteratively

// iteratively

? ListNode* reverseList(ListNode* head) {
? ? ? ? if(head == NULL || head->next == NULL)
???????? ? ?return head;

????????ListNode * pPrevious = NULL;

????????ListNode * pCurrent = head;

????????ListNode * pNext = head->next;

??????while( pNext){

??????????pCurrent->next = pPrevious;
? ? ? ? ? pPrevious = pCurrent;
??????????pCurrent = pNext;
? ? ? ? ? pNext = pNext->next;
??????????pCurrent->next = pPrevious;
???????????
????}

????return pCurrent;
? ? }

//?recursion

?ListNode* reverseList(ListNode* head) {
? ? ? ? if(head == NULL || head->next == NULL)
? ? ? ? ? ? return head;
? ? ? ? ListNode* last = reverseList(head->next);
? ? ? ? head->next->next = head;
? ? ? ? head->next = NULL;
? ? ? ? ? ??
? ? ? ? return last;
? ? }