反转链表 K个一组翻转链表

目录

LeetCode206 反转链表

LeetCode92 反转链表II

LeetCode25 K个一组翻转链表

LeetCode206 反转链表

/**
 * 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) {
        ListNode* pre=nullptr;
        ListNode* cur=head;
        while(cur){
            ListNode* nex=cur->next;
            cur->next=pre;
            pre=cur;
            cur=nex;
        }
        return pre;
    }
};

LeetCode92 反转链表II

/**
 * 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* reverseBetween(ListNode* head, int left, int right) {
        ListNode* dummy=new ListNode(0);
        dummy->next=head;
        ListNode* p=dummy;
        for(int i=0;i<left-1;i++) p=p->next;
        ListNode* cur=p->next;
        ListNode* pre=nullptr;
        for(int i=0;i<right-left+1;i++){
            ListNode* nex=cur->next;
            cur->next=pre;
            pre=cur;
            cur=nex;
        }
        p->next->next=cur;
        p->next=pre;
        return dummy->next;
    }
};

LeetCode25 K个一组翻转链表

/**
 * 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* reverseKGroup(ListNode* head, int k) {
        int n=0;
        for(ListNode* i=head;i;i=i->next) n++;
        ListNode* dummy=new ListNode(0);
        dummy->next=head;
        ListNode* p=dummy;
        ListNode* cur=head;
        for(;n>=k;n-=k){
            ListNode* pre=nullptr;
            for(int i=0;i<k;i++){
                ListNode* nex=cur->next;
                cur->next=pre;
                pre=cur;
                cur=nex;
            }
            ListNode* ne=p->next;
            p->next->next=cur;
            p->next=pre;
            p=ne;
        }
        return dummy->next;
    }
};