19. Remove Nth Node From End of List

考察点：链表，指针；
思路：过滤掉特殊情况，然后是通用情况；
C++ 代码：

/** * 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;        if (head->next == NULL && n == 1)            return NULL;        int len = 0;        ListNode * temp_len = head;        while (temp_len){            len++;            temp_len = temp_len->next;        }        if (n >=len) {            temp_len = head->next;            free(head);            return temp_len;        }// 长度等于n的情况        int i=0;        ListNode * after = head;        ListNode * before = head;        while (after->next) {            i++;            if (i == n) {                before = head;            }            if (i > n) {                before = before->next;            }            after = after->next;        }        ListNode * temp = before->next;        before->next = temp->next;        free(temp);        return head;    }};

discuss中的奇淫技巧：
代码：

class Solution{public:    ListNode* removeNthFromEnd(ListNode* head, int n)    {        ListNode** t1 = &head, *t2 = head;        for(int i = 1; i < n; ++i)        {            t2 = t2->next;        }        while(t2->next != NULL)        {            t1 = &((*t1)->next);            t2 = t2->next;        }        *t1 = (*t1)->next;        return head;    }};