LeetCode 234. 回文链表

我的解答

后序遍历

#include <bits/stdc++.h>
using namespace std;
/**
 * 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) {}
 * };
 */

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
{
    ListNode *left { nullptr };
public:
    bool isPalindrome(ListNode *head)
    {
        left = head;
        return postOrderTraverse(head);
    }
    bool postOrderTraverse(ListNode *right)
    {
        if (right)
        {
            bool res = postOrderTraverse(right->next) && (left->val == right->val);
            left = left->next;
            return res;
        }
        else
        {
            return true;
        }
    }
};

int main()
{

}
// g++ test.cpp --std=c++11 -pedantic -Wall -Wextra

中点、反转、节省空间

#include <bits/stdc++.h>
using namespace std;
/**
 * 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) {}
 * };
 */

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:
    bool isPalindrome(ListNode *head)
    {
        ListNode *lastHalfPtr = lastHalf(head);
        lastHalfPtr = reverse(lastHalfPtr);
        while (lastHalfPtr)
        {
            if (lastHalfPtr->val != head->val)
            {
                return false;
            }
            lastHalfPtr = lastHalfPtr->next;
            head = head->next;
        }
        return true;
    }
    /**
     * @brief 找到链表的后半段
     *
     * @param head
     * @return
     */
    ListNode* lastHalf(ListNode *head)
    {
        ListNode *fast = head, *slow = head;
        while (fast && fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        if (fast) // 链表长度为基数
        {
            return slow->next;
        }
        else // 链表长度为偶数
        {
            return slow;
        }
    }
    ListNode* reverse(ListNode *head)
    {
        ListNode *prev = nullptr, *cur = head, *next = nullptr;
        while (cur)
        {
            next = cur->next;
            cur->next = prev;

            prev = cur;
            cur = next;
        }
        return prev;
    }
};

int main()
{

}
// g++ test.cpp --std=c++11 -pedantic -Wall -Wextra