remove-nth-node-from-end-of-list.cpp

// Time:  O(n)
// Space: O(1)

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: The head of linked list.
     */
    ListNode *removeNthFromEnd(ListNode *head, int n) {
        ListNode dummy{0};
        dummy.next = head;
        auto slow = &dummy;
        auto fast = &dummy;

        // fast is n-step ahead.
        while (n > 0) {
            fast = fast->next;
            --n;
        }

        // When fast reaches the end, slow must be nth to last node.
        while (fast->next != nullptr) {
            slow = slow->next;
            fast = fast->next;
        }

        auto node_to_delete = slow->next;
        slow->next = slow->next->next;
        delete node_to_delete;

        return dummy.next;
    }
};