19. Remove Nth Node From End of List

Description

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.

Solution

题目要求只遍历一遍链表，并且保证了n的有效性，注意dummy的用法，简化删除头部节点的逻辑

ListNode* removeNthFromEnd(ListNode* head, int n) {
    ListNode* dummy = new ListNode(-1);
    dummy->next = head;
    auto first = dummy, second = dummy;
    while (n--) {
        first = first->next;
    }
    while (first->next) {
        first = first->next;
        second = second->next;
    }
    auto pNode = second->next;
    second->next = pNode->next;
    pNode->next = NULL;
    delete pNode;
    return dummy->next;
}