[LeetCode] 19 Remove Nth Node From End of List

判斷被刪掉是否是head

稍微有點繞

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
  const int MAGIC = 55668787;
  ListNode* removeNthFromEnd(ListNode* head, int n) {
    ListNode* slowNode = head;
    ListNode* prevNode = new ListNode(MAGIC, slowNode);
    ListNode* fastNode = head;
    
    // step 1 : find n-th node from the end
    for (int i = 0; i < n; i++)
      fastNode = fastNode->next;
    
    while (fastNode != nullptr) {
      prevNode = slowNode;
      slowNode = slowNode->next;
      fastNode = fastNode->next;
    }
    // Step 2 : Remove slowNode  
    //          In this moment, slowNode is the n-th node from the end
    prevNode->next = slowNode->next;
    if (slowNode == head) 
      if (slowNode->next == nullptr)
        return nullptr;
      else
        return prevNode->next;
    else
      return head;
  }
};