【力扣】141. 环形链表、160. 相交链表、206.反转链表、234. 回文链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head)
{
    if (head == NULL || head->next == NULL) 
    {
        return false;
    }
 
    struct ListNode *slow = head, *fast = head; // 定义慢、快两个指针

    do {
        if(fast == NULL || fast->next == NULL) // 链表无环退出返回 false
        {
            return false;
        }
        slow = slow->next;          // 慢指针一次移动一个节点
        fast = fast->next->next;    // 快指针一次移动两个节点
    } while(slow != fast);          // 节点相等退出循环

    return true;
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode *getIntersectionNode(struct ListNode *headA, struct ListNode *headB) {
    int lenA = 0, lenB = 0;     // 用于获取两个链表长度
    struct ListNode *pA = headA, *pB = headB;
    int i = 0;

    while(pA != NULL)   // 获取链表 A 长度
    {
        lenA++;
        pA = pA->next;
    }

    while(pB != NULL)   // 获取链表 B 长度
    {
        lenB++;
        pB = pB->next;
    }

    pA = headA;
    pB = headB;

    // 对齐链表
    if(lenA > lenB)
    {
        for(i = 0; i < lenA - lenB; i++)
        {
            pA = pA->next;
        }
    }
    else
    {
        for(i = 0; i < lenB - lenA; i++)
        {
            pB = pB->next;
        }
    }

    // 查找相交节点
    while(pA != NULL)
    {
        if(pA != pB)
        {
            pA = pA->next;
            pB = pB->next;
        }
        else
        {
            return pA;
        }
    }
    return NULL;
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* reverseList(struct ListNode* head) {
    struct ListNode *newhead = NULL;
    struct ListNode *p = head;

    while(p != NULL)
    {
        struct ListNode *q = p->next; // 存储原链表第二个节点
        p->next = newhead;  // 将原链表头节点添加到新链表，成为新链表头节点
        newhead = p;        // 更新新链表头节点
        p = q;              // 更新原链表头节点
    }
    return newhead;
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* reverseList(struct ListNode* head) {
    if (head == NULL || head->next == NULL) {
        return head;
    }
    struct ListNode* newHead = reverseList(head->next);
    head->next->next = head;
    head->next = NULL;
    return newHead;
}

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool isPalindrome(struct ListNode* head) {
    int num[100000] = {0};      // 用来存放数据
    int n = 0;                  // 计算列表长度

    while(head != NULL)         // 遍历链表
    {
        num[n++] = head->val;   // 将数据更新到数组中
        head = head->next;
    }

    for(int i = 0, j = n - 1; i < j; i++, j--)
    {
        if(num[i] != num[j])    // 不满足回文
        {
            return false;
        }
    } 
    return true;
}

/**
 * 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:
    bool isPalindrome(ListNode* head) {

        ListNode *p = head;
        stack<int> val_stack;       // 创建一个栈

        while(p != NULL)
        {
            val_stack.push(p->val); // 将链表数据依次入栈
            p = p->next;
        }

        p = head;                   // 从头遍历链表
        while(p != NULL)
        {
            if(p->val != val_stack.top()) // 对比链表数据和栈顶元素
            {
                return false;
            }
            val_stack.pop();        // 移除栈顶元素
            p = p->next;
        }
        return true;
    }
};