LeetCode【234.回文链表】

版权声明： https://blog.csdn.net/qq_38386316/article/details/83017044

题目描述

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： $true$

思路 * 1
使用数组存储所有结点的值，然后利用一个循环进行值的比较就可以了。

代码 * 1

class Solution {
    public boolean isPalindrome(ListNode head) {
        if(head==null||head.next==null) return true;
        int[] a= new int[50000];
        int i=0;
        while(head!=null){
            a[i++]=head.val;
            head=head.next;
        }
        boolean flag=true;
        for(int j=0;j<i/2;j++){
            if(a[j]!=a[i-j-1]){
                flag=false;
                break;
            }    
        }
        return flag; 
    }
}

---

思路 * 2
使用快慢指针的思路首先求出中间结点 偶数个时，偏右的那个结点,把从slow指针的结点开始的链表进行翻转.
然后让反转后的链表和原始链表的前半段进行值的比较，不同的话直接返回false,否则返回true。

代码 * 2

class Solution {
    public boolean isPalindrome(ListNode head){
        ListNode fast = head, slow = head;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        slow = reverse(slow);
        fast = head;
        while(slow != null) {
            if(fast.val != slow.val) {
                return false;
            }
            fast = fast.next;
            slow = slow.next;
        }
        return true;
    }
    public ListNode reverse(ListNode head){
        ListNode prev = null;
        while(head != null) {
            ListNode nextNode = head.next;
            head.next = prev;
            prev = head;
            head = nextNode;
        }
        return prev;
    }
}

复杂度分析

• 时间复杂度： $O(N)$
• 空间复杂度： $O(1)$

完整代码

package leetcode234;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 * Created by 张超帅 on 2018/10/11.
 */
class ListNode {
    int val;
    ListNode next;
    ListNode(int val) { this.val = val;}
}
class Solution {
    public boolean isPalindrome(ListNode head){
        ListNode fast = head, slow = head;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        slow = reverse(slow);
        fast = head;
        while(slow != null) {
            if(fast.val != slow.val) {
                return false;
            }
            fast = fast.next;
            slow = slow.next;
        }
        return true;
    }
    public ListNode reverse(ListNode head){
        ListNode prev = null;
        while(head != null) {
            ListNode nextNode = head.next;
            head.next = prev;
            prev = head;
            head = nextNode;
        }
        return prev;
    }
}
public class leetcode234 {
    public static int[] stringToArrays(String input) {
       input = input.trim();
       input = input.substring(1, input.length() - 1);
       if(input == null) {
           return new int[0];
       }
       String[] parts = input.split(",");
       int[] res = new int[parts.length];
       for(int i = 0; i < parts.length; i ++) {
           res[i] = Integer.parseInt(parts[i].trim());
       }
       return res;
    }
    public static ListNode stringToListNode(String input) {
        int[] nodes = stringToArrays(input);
        ListNode dummpy = new ListNode(-1);
        ListNode cur = dummpy;
        for(int node : nodes) {
            cur.next = new ListNode(node);
            cur = cur.next;
        }
        return dummpy.next;
    }

    public static void main(String[] args) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String line = null;
        while ((line = in.readLine()) != null) {
            ListNode head = stringToListNode(line);
            Boolean result = new Solution().isPalindrome(head);
            System.out.println(result);
        }
    }
}