solved 'Remove Nth Node From End of List' on LeetCode https://leetcode.com/problems/remove-nth-node-from-end-of-list/

remove_nth_one_pass.cc

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
 
// Very clever algorithm, from
// https://leetcode.com/discuss/21104/simple-java-solution-in-one-pass
// Use a fast runner to pinpoint the right place for deletion in one pass.
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode *fast = head, *slow = head;
        for (int i = 0; i < n; ++i) {
            fast = fast->next;
        }
        if (!fast) {
            // Delete the head.
            ListNode *res = head->next;
            delete head;
            return res;
        }
        
        // Find the right place to delete.
        for (fast = fast->next; fast; fast = fast->next, slow = slow->next) ;

        delete slow->next;
        slow->next = slow->next->next;
        return head;
    }
};