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.

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
struct ListNode* removeNthFromEnd(struct ListNode* head, int n) {
struct ListNode* p1 = head;
struct ListNode* p2 = head;

for(int i = 0; i < n; i++){
p1 = p1->next;
}
if(!p1){ //special condition: delete head
head = head->next;
free(p2);
return head;
}

while(p1->next){
p1 = p1->next;
p2 = p2->next;
}

p1 = p2->next;
p2->next = p2->next->next;
free(p1);
return head;
}