关键词：快慢指针，反转链表
代码和注释:
/**
* 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 {
// rightMiddle is the node next of the middle node;
ListNode rightMiddle = null;
public boolean isPalindrome(ListNode head) {
// If list has 0/1 node, return true;
if (head == null || head.next==null){
return true;
}
// Find the rightMiddle node, using quick/slow pointers;
findRightMiddle(head);
// Reverse the list after rightMiddle node, and return the new head;
rightMiddle = reverse(rightMiddle);
// Compare two list
while (rightMiddle != null){
if (head.val != rightMiddle.val){
return false;
} else {
head = head.next;
rightMiddle = rightMiddle.next;
}
}
return true;
}
private ListNode reverse(ListNode head){
ListNode cur=head, pre=null, nxt= null;
while (cur != null ){
nxt = cur.next;
cur.next = pre;
pre = cur;
cur = nxt;
}
return pre;
}
private void findRightMiddle(ListNode head){
ListNode fast = head;
ListNode slow = head;
rightMiddle = slow.next;
while (fast.next!=null && fast.next.next!=null){
slow = rightMiddle;
rightMiddle = slow.next;
fast = fast.next.next;
}
}
}
测试用例:
[1,2,2,1]
[1,2]
[1,2,3,2,1]
[1,2,2,2,3,1]
[1,2,3,1,1]
[1]
可能的改进:
Use rightMiddle to replace slow completely in function findRightMiddle()
