19 Remove Nth Node from End of the List

使用快慢指针的方法，维护两个指针，指针的间距是N， 当快指针为空的时候，慢指针指向了需要删除的节点。

两个技巧，使用dummy头在全部的元素之前，另一个技巧将前面的过程合并成一个循环来写。

Python

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def removeNthFromEnd(self, head, n):
        """
        :type head: ListNode
        :type n: int
        :rtype: ListNode
        """
        dummy = ListNode(0)
        dummy.next = head
        fast, slow = dummy, dummy
        while fast.next:
            if n <= 0:
                slow = slow.next
            fast = fast.next
            n -= 1
        if slow.next is not None:
            slow.next = slow.next.next
        return dummy.next

Java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode slow = dummyHead;
        ListNode fast = dummyHead;
        while(fast.next != null){
            if (n <= 0){
                slow = slow.next;
            }
            fast = fast.next;
            n --;
        }
        if(slow.next != null){
            slow.next = slow.next.next;
        }
        return dummyHead.next;
    }
}