LeetCode 19. 删除链表的倒数第 N 个结点

Posted 2021-06-27 Blocking The Sky

题目描述

给你一个链表，删除链表的倒数第 n 个结点，并且返回链表的头结点。

进阶：你能尝试使用一趟扫描实现吗？

代码

1、双指针

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        ListNode *first = new ListNode(0);
        ListNode *second = new ListNode(0);
        ListNode *prev = new ListNode(0);
        int k=0;
        first=head;
        second=head;
        while(k<n){
            first=first->next;
            k++;
        }
        while(first){
            first=first->next;
            prev=second;
            second=second->next;
        }
        if(second==head){
            return head->next;
        }
        prev->next=second->next;
        return head;
    }
};

2、一次遍历加哈希

/**
 * 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* removeNthFromEnd(ListNode* head, int n) {
        unordered_map<int,ListNode *> hashmap;
        ListNode *p=new ListNode(0,head);
        int sum=0;
        p=p->next;
        while(p){
            sum++;
            hashmap[sum]=p;
            //cout<<p->val;
            p=p->next;
        }
        //cout<<n<<sum;
        if(n==sum){
            if(n>1){
                head=head->next;
                //cout<<head->val;
                return head;
            }
            else 
                return NULL;
        }
        else{
            hashmap[sum-n]->next=hashmap[sum-n+1]->next;
        }
        return head;
    }
};