234. 回文链表

categories: [Blog,Algorithm]

234. 回文链表

难度简单872
请判断一个链表是否为回文链表。
示例 1:
输入: 1->2
输出: false
示例 2:
输入: 1->2->2->1
输出: true

进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        List<Integer> vals = new ArrayList<Integer>();
        // 将链表的值复制到数组中
        ListNode currentNode = head;
        while (currentNode != null) {
            vals.add(currentNode.val);
            currentNode = currentNode.next;
        }
        // 使用双指针判断是否回文
        int front = 0;
        int back = vals.size() - 1;
        while (front < back) {
            if (!vals.get(front).equals(vals.get(back))) {
                return false;
            }
            front++;
            back--;
        }
        return true;
    }
// 作者：LeetCode-Solution
// 链接：https://leetcode-cn.com/problems/palindrome-linked-list/solution/hui-wen-lian-biao-by-leetcode-solution/
}

栈

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        Stack<Integer> stk =new Stack();
        if (null==head || null==head.next) return true;
        ListNode p = head;
        while (p != null) {
            stk.push(p.val);
            p = p.next;
        }
        while (head != null){
            if(head.val == stk.peek()) {
                stk.pop();
            } else return false;
            head = head.next;
        }
        return true;
    }
// 作者：wo-yao-chu-qu-luan-shuo
// 链接：https://leetcode-cn.com/problems/palindrome-linked-list/solution/hui-wen-lian-biao-fu-zhu-zhan-by-wo-yao-ab2uc/
}