NC96 判断一个链表是否为回文结构

 分析：

1、反转链表，然后对比，空间复杂度O(n)，时间复杂度O(n)

2、将链表数据存到数组里，时间复杂度空间复杂度都是O(n)

3、存到栈里

4、每次对比从前到后找到指定位置，时间复杂度O(n^2)，空间复杂度O(1)——超时

1. 反转全部链表

import java.util.*;



public class Solution {
    
    public boolean isPail (ListNode head) {
        // write code here
        if(head == null||head.next == null) return true;
        ListNode index = head;
        ListNode thead = new ListNode(head.val);
        ListNode p = thead;
        while(index.next != null){
            p.next = new ListNode(index.next.val);
            index = index.next;
            p = p.next;
        }
        p = reverseList(thead);
        index = head;
        while(index != null && p != null){
            if(index.val != p.val) return false;
            index = index.next;
            p = p.next;
        }
        return true;
    }
    public ListNode reverseList(ListNode head){
        if(head == null||head.next == null) return head;
        ListNode curr = null;
        ListNode last = null;
        ListNode next = head;
        while(next != null){
            last = curr;
            curr = next;
            next = next.next;
            curr.next = last;

        }
        return curr;
    }
}

 反转后半部分链表

import java.util.*;



public class Solution {
    
    public boolean isPail (ListNode head) {
        // write code here
        if(head == null||head.next == null) return true;
        ListNode slow = head;
        ListNode fast = head;
        ListNode p;
        ListNode index;
        while(fast != null && fast.next != null){//快慢指针找中间位置
            fast = fast.next.next;
            slow = slow.next;
        }
        if(fast != null) slow = slow.next;//原链表长度为奇数时，慢指针往后移一位，将中间一位错开
        
        p = reverseList(slow);
        index = head;
        while(index != null && p != null){
            if(index.val != p.val) return false;
            index = index.next;
            p = p.next;
        }
        return true;
    }
    public ListNode reverseList(ListNode head){
        if(head == null||head.next == null) return head;
        ListNode curr = null;
        ListNode last = null;
        ListNode next = head;
        while(next != null){
            last = curr;
            curr = next;
            next = next.next;
            curr.next = last;

        }
        return curr;
    }
}

2. 放数组里

import java.util.*;
 

 
public class Solution {
    
    public boolean isPail (ListNode head) {
        // write code here
        if(head == null||head.next == null)return true;
        int len = 0;
        ListNode index = head;
        while(index != null){
            index = index.next;
            len++;
        }
        int[] tmp = new int[len];
        index = head;
        for(int i = 0;i < len;i++){
            tmp[i] = index.val;
            index = index.next;
        }
        for(int i = 0; i < len/2;i++){
            if(tmp[i]!=tmp[len-i-1]) return false;
        }
        return true;
    }
}

3. 全部入栈

import java.util.*;



public class Solution {
    
    public boolean isPail (ListNode head) {
        // write code here
        ListNode index = head;
        Stack stack = new Stack();
        while(index != null){
            stack.push(index.val);
            index = index.next;
        }
        index = head;
        while(stack.size() != 0){
            if(stack.pop() != index.val) return false;
            index = index.next;
        }
        return true;
    }
}

4.

import java.util.*;



public class Solution {
    
    public boolean isPail (ListNode head) {
        // write code here
        if(head == null || head.next == null) return true;
        ListNode index = head;
        ListNode t = head;
        int len = 0;
        while(index != null){
            len++;
            index = index.next;
        }
        index = head;
        int cnt = 0;
        for(int i = 1;i <= len/2;i++){
            int key = index.val;
            cnt = len + 1 - i;
            t = head;
            while(cnt != 1){
                t = t.next;
                cnt--;
            }
            if(key != t.val)
                return false;
            index = index.next;
        }
        return true;
    }
}

提交结果：运行超时 运行时间：8001ms 占用内存：132128KB 使用语言：Java 用例通过率：84.62%