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?
** 解题思路 **
（1）用快慢指针找到中点
（2）然后把右边那一半反转。
（3）再依次比较左右两边，如果不相等，则返回false； 若左右两边对称相等，则返回true， 即这个链表是回文链表。
This can be solved by reversing the 2nd half and compare the two halves. Let's start with an example [1, 1, 2, 1].
In the beginning, set two pointers fast and slow starting at the head.
1 -> 1 -> 2 -> 1 -> null
sf
(1) Move: fast pointer goes to the end, and slow goes to the middle.
1 -> 1 -> 2 -> 1 -> null
s f
(2) Reverse: the right half is reversed, and slow pointer becomes the 2nd head.
1 -> 1 null <- 2 <- 1
h s
(3) Compare: run the two pointers head and slow together and compare.
1 -> 1 null <- 2 <- 1
h s
** 参考代码 **
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
public class IsPalindrome_234 {
public boolean isPalindrome(ListNode head) {
// ref" https://leetcode.com/discuss/45656/easy-understand-java-solution-o-1-space-cost
if(head == null) {
return true;
}
ListNode p1 = head;
ListNode p2 = head;
ListNode p3 = p1.next;
ListNode pre = p1;
//find mid pointer, and reverse head half part
while(p2.next != null && p2.next.next != null) {
p2 = p2.next.next;
pre = p1;
p1 = p3;
p3 = p3.next;
p1.next = pre;
}
//odd number of elements, need left move p1 one step
if(p2.next == null) {
p1 = p1.next;
}
else { //even number of elements, do nothing
}
//compare from mid to head/tail
while(p3 != null) {
if(p1.val != p3.val) {
return false;
}
p1 = p1.next;
p3 = p3.next;
}
return true;
}
}
}
