链表专题

思路：新建一个temp节点，双指针后移

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null||head.next==null){
return head; }
        ListNode a=head;
        ListNode b=head.next;
        while(b!=null)
        {
            ListNode c=b.next;
            b.next=a;
            a=b;
            b=c;
            
        }
        head.next=null;
        return a;
        
        
        
    }
}

View Code

2、链表内指定区间反转

思路：1、先将待反转的区域反转

2、把 pre 的 next 指针指向反转以后的链表头节点，把反转以后的链表的尾节点的 next 指针指向 succ

/**
 * 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(-1);
                dummy->next=head;
                auto pre=dummy;
                for(int i=0;i<left-1;i++)
                {
                    pre=pre->next;
                }
                
                auto cur=pre->next;
                auto leftNode=pre->next;
                for(int i=left;i<right;i++)
                {
                   cur=cur->next;
                }
                auto next=cur->next;
                cur->next=NULL;
                pre->next=NULL;

                fun(leftNode);
            pre->next=cur;
            leftNode->next=next;
            return dummy->next;




    }
    void fun(ListNode* &head)
    {
        auto a=head;
        auto b=head->next;
        while(b)
        {
            auto c=b->next;
            b->next=a;
            a=b;
            b=c;
        }
        head->next=nullptr;
    }
};

View Code

3、合并k个已排序的链表

思路：

class Solution {
public:
    ListNode* mergeTwoLists(ListNode *a, ListNode *b) {
        if ((!a) || (!b)) return a ? a : b;
        ListNode head, *tail = &head, *aPtr = a, *bPtr = b;
        while (aPtr && bPtr) {
            if (aPtr->val < bPtr->val) {
                tail->next = aPtr; aPtr = aPtr->next;
            } else {
                tail->next = bPtr; bPtr = bPtr->next;
            }
            tail = tail->next;
        }
        tail->next = (aPtr ? aPtr : bPtr);
        return head.next;
    }

    ListNode* merge(vector <ListNode*> &lists, int l, int r) {
        if (l == r) return lists[l];
        if (l > r) return nullptr;
        int mid = (l + r) >> 1;
        return mergeTwoLists(merge(lists, l, mid), merge(lists, mid + 1, r));
    }

    ListNode* mergeKLists(vector<ListNode*>& lists) {
        return merge(lists, 0, lists.size() - 1);
    }
};

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/merge-k-sorted-lists/solution/he-bing-kge-pai-xu-lian-biao-by-leetcode-solutio-2/
来源：力扣（LeetCode）
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