Description:

题目大意：删除单项链表的倒数第n个节点

解题思路:

算法标签：双指针

1. 设置虚拟头节点
2. 设置双指针 pre 和 last，使其两个指针相隔 n 个元素
3. 双指针同时右移，直到 last 为 NULL
4. 删除 pre 的下一个元素
5. 返回 head

代码:

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

class Solution {
public:
    ListNode* removeNthFromEnd(ListNode* head, int n) {
        ListNode* dummyhead = new ListNode(0);
        dummyhead -> next = head;
        
        ListNode* pre = dummyhead;
        ListNode* last = dummyhead;
        
        for(int i = 0;i < n+1;i++) 
            last = last -> next;
        while(last != NULL) {
            pre = pre -> next;
            last = last  -> next;
        }
        
        ListNode* delNode = pre -> next;
        pre -> next = delNode -> next;
        delete delNode;
        return dummyhead -> next;
    }
};