【LeetCode】019. 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.

题解：

　　快慢指针。

/**
 * 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) {
        if (!head)
            return head;
        ListNode dummy(-1);
        dummy.next = head;
        ListNode* fast = &dummy, *slow = &dummy;
        
        while (n-- >= 0 && fast) fast = fast->next;
        
        while(fast) {
            fast = fast->next;
            slow = slow->next;
        }
        
        ListNode* tmp = slow->next;
        slow->next = slow->next->next;
        delete tmp;
        
        return dummy.next;
    }
};