1 链表常规题目

链表常规题目：

2 链表反序

• 初始化：单链表需要指向一个前向节点：
  
• 反序：
  

2.1 代码示例

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode *pre = NULL, *cur = head;
        while(cur)
        {
            ListNode *next = cur->next;
            cur->next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
};

注意中间4步：https://leetcode-cn.com/problems/fan-zhuan-lian-biao-lcof/

3 回文链表

1. 利用快慢指针的思想：

   • 快指针每次走两格，慢指针每次走一格，则快指针到终点时候，慢指针刚好在中间位置
   • 反转后半部分链表：
   • 比较值

   class Solution {
   
       public boolean isPalindrome(ListNode head) {
           // 快慢指针找中点
           ListNode slow = head;
           ListNode fast = head;
           while (fast != null && fast.next != null) {
               slow = slow.next;
               fast = fast.next.next;
           }
           // 反转后半部分
           ListNode pre = null;
           while (slow != null) {
               ListNode next = slow.next;
               slow.next = pre;
               pre = slow;
               slow = next;
           }
           // 前后两段比较是否一致
           ListNode node = head;
           while (pre != null) {
               if (pre.val != node.val) {
                   return false;
               }
               pre = pre.next;
               node = node.next;
           }
           return true;
       }
   }
   

2. 递归思想

   class Solution {
   public:
       ListNode *root;
       bool flag = true;
       void dfs(ListNode *head){
   
           if(head == NULL){
               return ;
           }
           dfs(head->next);
           if(head->val != root->val){
               flag = false;
           }
           root = root->next;
       }
       bool isPalindrome(ListNode* head) {
           root = head;
           dfs(head);
           return flag;
       }
   };
   

注意：https://leetcode-cn.com/problems/palindrome-linked-list-lcci/comments/