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.

思路1：双指针操作，一个快一个慢。O(n)

public class Solution {
    public ListNode removeNthFromEnd(ListNode head, int n) {
       ListNode slow = head;
       ListNode fast = head;

       while(n>0){
           fast = fast.next;
           n--;
       }

       if (fast==null){
           head = head.next;
           return head;
       }

       while(fast.next!=null){
           fast = fast.next;
           slow = slow.next;
       }

       slow.next = slow.next.next;
       return head;
    }
}

C++ code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* slow = head;
        ListNode* fast = head;

        while (n>0){
            fast = fast->next;
            n--;
        }
        if (!fast){
            head = head->next;
            return head;
        }

        while(fast->next){
            fast = fast->next;
            slow = slow->next;
        }

        slow->next = slow->next->next;
        return head;
    }
};