19. Remove Nth Node From End of List

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.

题意：删除链表的从尾部数第n个元素。n始终有效，要求只能遍历一遍。

思路：用两个指针，间隔n同时向后走，当最后一个指针走向尾部时，开始删除，注意删除的是head结点的情况。

class Solution {
public:
ListNode* removeNthFromEnd(ListNode* head, int n) {
if (n == 0)
return head;
ListNode *p, *q;
p = q = head;
while (n--){
q = q->next;
}
if (q){
while (q->next){
p = p->next;
q = q->next;
}
q = p->next->next;
free(p->next);
p->next = q;
}
else{
head = p->next;
free(p);
}
return head;
}
};