LCR 024. 反转链表

LCR 024. 反转链表

头插法

/**
 * Definition for singly-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) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(!head)
            return NULL;
        auto dummy = new ListNode(-1);

        while(head){
            auto p = head->next;
            head->next = dummy->next;
            dummy->next = head;
            head = p;
        }
        return dummy->next;
    }
}

迭代

    ListNode* reverseList(ListNode* head) {
        if(!head || !head->next)
            return head;
        
        auto a = head;
        auto b = head->next;
        auto c = b->next;
        while(b){
            c = b->next;
            b->next = a;
            a = b ;
            b = c;
        }
        head->next = NULL;
        return a;
    }