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IsPalindromeList234.java
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IsPalindromeList234.java
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package Leetcode;
public class IsPalindromeList234 {
/*Method : Two pointers, Time: O(N), Space:O(1)**/
public boolean isPalindrome(ListNode head) {
if(head == null) {
return true;
}
ListNode fistHalfEnd = getEndOfFirstHalf(head);
ListNode secondHalfStart = reverseList(fistHalfEnd.next);
ListNode curFirst = head;
ListNode curSecond = secondHalfStart;
boolean result = true;
while (result && curSecond != null ) {
if(curFirst.val != curSecond.val) {
result = false;
}
curFirst = curFirst.next;
curSecond = curSecond.next;
}
fistHalfEnd.next = reverseList(secondHalfStart);
return result;
}
private ListNode reverseList(ListNode head) {
ListNode cur = head;
ListNode pre = null;
while (cur != null) {
ListNode temp = cur.next;
cur.next = pre;
pre = cur;
cur =temp;
}
return pre;
}
private ListNode getEndOfFirstHalf(ListNode head) {
ListNode first = head;
ListNode second = head;
while(second.next != null && second.next.next != null) {
first = first.next;
second =second.next.next;
}
return first;
}
//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; }
}
}