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;
}