19. Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

For example,

 Given linked list: 1->2->3->4->5, and n = 2.

 After removing the second node from the end, the linked list becomes 1->2->3->5.

Note: Given n will always be valid. Try to do this in one pass.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode dummy(0);
        dummy.next = head;
        ListNode *slow = &dummy;
        ListNode *fast = &dummy;

        for(int i = 0; i < n; i++){
            fast = fast->next;
        }

        while(fast->next != NULL){
            fast = fast->next;
            slow = slow->next;
        }
        auto next = slow->next;
        slow->next = slow->next->next;
        delete next;

        return dummy.next;
    }
};

二级指针

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode **pHead = &head;
        ListNode *fast = head;
        for(int i = 1; i < n; i++)
            fast = fast->next;

        while(fast->next){
            fast = fast->next;
            pHead = &((*pHead)->next);
        }

        *pHead = (*pHead)->next;
        return head;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        if(head == nullptr || n < 1)   return head;

        ListNode *cur = head;

        while(cur != nullptr){
            n--;
            cur = cur->next;
        }

        if(n == 0){
            head = head->next;
        }
        if(n < 0){
            cur = head;
            while(++n != 0){
                cur = cur->next;
            }
            cur->next = cur->next->next;
        }
        return head;
    }
};