234. 回文链表 Palindrome Linked List

Posted 2020-09-07 Long Long Journey

Given a singly linked list, determine if it is a palindrome.

Follow up:
Could you do it in O(n) time and O(1) space?

判断一个链表是否为回文串

思路：1.找到中间点，2.反转后半部分链表，3.判断前半部分与后半部分是否相同

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
public class Solution {
	public ListNode ReverseList(ListNode head) {
		if (head == null || head.next == null)
			return head;
		ListNode nextNode = head.next;
		ListNode newHead = ReverseList(nextNode);
		nextNode.next = head;
		head.next = null;
		return newHead;
	}
	public ListNode FindMid(ListNode head) {
		ListNode fast = head;
		ListNode slow = head;
		int num = 1;
		while (fast != null) {
			num++;
			if (fast.next != null) {
				fast = fast.next.next;
			} else {
				break;
			}
			slow = slow.next;
		}
		return slow;
	}
    
    public bool IsPalindrome(ListNode head) {
		if (head == null) {
			return true;
		}
		ListNode midNode = FindMid(head);
		ListNode newMid = ReverseList(midNode);
		ListNode font = head;
		ListNode back = newMid;
		while (back != null) {
			if (font.val != back.val) {
				return false;
			}
			font = font.next;
			back = back.next;
		}
		return true;
    }
}

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