19. Remove Nth Node From End of List

O(n)

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
        dummy = ListNode(next = head)
        slow, fast = dummy, head
        for _ in range(n):
            fast = fast.next
        while fast:
            slow, fast = slow.next, fast.next
        slow.next = slow.next.next
        return dummy.next

/**
 * 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_head = 0;
        dummy_head.next = head;
        auto fast = head, slow = &dummy_head; // slow指向head的前一个，这样最后slow指向要删的node的前一个，方便删除，比如[1]删倒数第1
        while (n-- > 0) {
            fast = fast->next;
        }
        while (fast) {
            slow = slow->next;
            fast = fast->next;
        }
        slow->next = slow->next->next;
        return dummy_head.next;
    }
};

/**
 * 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_head = 0;
        dummy_head.next = head;
        auto fast = head, slow = &dummy_head;
        while (fast) {
            fast = fast->next;
            if (--n < 0) {
                slow = slow->next;
            }
        }
        slow->next = slow->next->next;
        return dummy_head.next;
    }
};

/**
 * 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) {
        // write your code here
        ListNode dummy_head(0);
        dummy_head.next = head;
        ListNode *fast = head;
        while (n-- > 0 && fast) {
            fast = fast->next;
        }
        if (n >= 0) return head; // 要考虑链表长度不足n
        ListNode *slow = head, *prev = &dummy_head;
        while (fast) {
            prev = prev->next;
            slow = slow->next;
            fast = fast->next;
        }
        prev->next = slow->next;
        return dummy_head.next;
    }
};

/**
 * 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)
            return head;
        auto front(head);
        auto behind(&head);
        while (front) {
            front = front->next;
            if (--n < 0)
                behind = &(*behind)->next;
        }
        auto to_be_deleted(*behind);
        *behind = (*behind)->next;
        delete to_be_deleted;
        return head;
    }
};