19. Remove Nth Node From End of List

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

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.

Follow up:

Could you do this in one pass?

// Two pointers
ListNode* removeNthFromEnd(ListNode* head, int n) { // time: O(n); space: O(1)
    if (!head) return nullptr;
    ListNode* dummy = new ListNode(-1e6);
    dummy->next = head;
    ListNode *fast = dummy, *slow = dummy;
    for (int i = 1; i <= n; ++i) fast = fast->next;
    while (fast->next != nullptr) {
        fast = fast->next;
        slow = slow->next;
    }
    ListNode* nodeToDelete = slow->next;
    slow->next = slow->next->next;
    delete nodeToDelete;
    ListNode* res = dummy->next;
    delete dummy;
    return res;
}