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Question
Formatted question description: https://leetcode.ca/all/234.html
Given the head
of a singly linked list, return true
if it is a palindrome or false
otherwise.
Example 1:
Input: head = [1,2,2,1] Output: true
Example 2:
Input: head = [1,2] Output: false
Constraints:
- The number of nodes in the list is in the range
[1, 105]
. 0 <= Node.val <= 9
Follow up: Could you do it in O(n)
time and O(1)
space?
Algorithm
The principle of using fast and slow pointers to find the midpoint,each time the fast pointer moves two steps and the slow pointer moves one step. When the fast pointer finishes, the position of the slow pointer is the midpoint.
After finding the midpoint, flip the linked list in the second half to compare in the order of palindrome.
Code
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public class Palindrome_Linked_List { /** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */ public boolean isPalindrome(ListNode head) { if (head == null) { return false; } // 2 pointers to find mid ListNode slowPrev = new ListNode(0); ListNode slow = head; ListNode fast = head; while (fast != null && fast.next != null) { fast = fast.next.next; slowPrev = slow; slow = slow.next; } // now slow aat middle slowPrev.next = null; // cut ListNode reversed2ndHalf = reverseNodeList(slow); ListNode p1 = head; ListNode p2 = reversed2ndHalf; while (p1 != null && p2 != null) { if (p1.val != p2.val) { return false; } p1 = p1.next; p2 = p2.next; } return true; } private ListNode reverseNodeList(ListNode head) { ListNode dummy = new ListNode(0); ListNode current = head; while (current != null) { ListNode currenHead = dummy.next; ListNode nextHead = current.next; current.next = null; // cut current node out dummy.next = current; current.next = currenHead; current = nextHead; } return dummy.next; } } ############ /** * 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 { public boolean isPalindrome(ListNode head) { ListNode slow = head; ListNode fast = head.next; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; } ListNode cur = slow.next; slow.next = null; ListNode pre = null; while (cur != null) { ListNode t = cur.next; cur.next = pre; pre = cur; cur = t; } while (pre != null) { if (pre.val != head.val) { return false; } pre = pre.next; head = head.next; } return true; } }
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// OJ: https://leetcode.com/problems/palindrome-linked-list/ // Time: O(N) // Space: O(1) class Solution { private: int getLength(ListNode *head) { int len = 0; for (; head; head = head->next, ++len); return len; } ListNode *reverse(ListNode *head) { ListNode dummy(0); while (head) { ListNode *node = head; head = head->next; node->next = dummy.next; dummy.next = node; } return dummy.next; } public: bool isPalindrome(ListNode* head) { if (!head) return true; int len = (getLength(head) + 1) / 2; ListNode *p = head, *q; while (--len > 0) p = p->next; q = p->next; p->next = NULL; q = reverse(q); while (head && q) { if (head->val != q->val) return false; head = head->next; q = q->next; } return true; } };
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# Definition for singly-linked list. # class ListNode: # def __init__(self, val=0, next=None): # self.val = val # self.next = next class Solution: def isPalindrome(self, head: Optional[ListNode]) -> bool: slow, fast = head, head.next while fast and fast.next: slow, fast = slow.next, fast.next.next pre, cur = None, slow.next while cur: t = cur.next cur.next = pre pre, cur = cur, t while pre: if pre.val != head.val: return False pre, head = pre.next, head.next return True ############ # Definition for singly-linked list. # class ListNode(object): # def __init__(self, x): # self.val = x # self.next = None class Solution(object): def isPalindrome(self, head): """ :type head: ListNode :rtype: bool """ def reverseList(root): pre = None cur = root while cur: tmp = cur.next cur.next = pre pre = cur cur = tmp return pre slow = fast = head while fast and fast.next: slow = slow.next fast = fast.next.next newHead = reverseList(slow) p1 = head p2 = newHead while p1 and p2: if p1.val != p2.val: return False p1 = p1.next p2 = p2.next return True
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/** * Definition for singly-linked list. * type ListNode struct { * Val int * Next *ListNode * } */ func isPalindrome(head *ListNode) bool { slow, fast := head, head.Next for fast != nil && fast.Next != nil { slow, fast = slow.Next, fast.Next.Next } var pre *ListNode cur := slow.Next for cur != nil { t := cur.Next cur.Next = pre pre = cur cur = t } for pre != nil { if pre.Val != head.Val { return false } pre, head = pre.Next, head.Next } return true }
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/** * Definition for singly-linked list. * class ListNode { * val: number * next: ListNode | null * constructor(val?: number, next?: ListNode | null) { * this.val = (val===undefined ? 0 : val) * this.next = (next===undefined ? null : next) * } * } */ function isPalindrome(head: ListNode | null): boolean { let slow: ListNode = head, fast: ListNode = head.next; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; } let cur: ListNode = slow.next; slow.next = null; let prev: ListNode = null; while (cur != null) { let t: ListNode = cur.next; cur.next = prev; prev = cur; cur = t; } while (prev != null) { if (prev.val != head.val) return false; prev = prev.next; head = head.next; } return true; }
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/** * Definition for singly-linked list. * function ListNode(val, next) { * this.val = (val===undefined ? 0 : val) * this.next = (next===undefined ? null : next) * } */ /** * @param {ListNode} head * @return {boolean} */ var isPalindrome = function (head) { let slow = head; let fast = head.next; while (fast && fast.next) { slow = slow.next; fast = fast.next.next; } let cur = slow.next; slow.next = null; let pre = null; while (cur) { let t = cur.next; cur.next = pre; pre = cur; cur = t; } while (pre) { if (pre.val !== head.val) { return false; } pre = pre.next; head = head.next; } return true; };
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/** * Definition for singly-linked list. * public class ListNode { * public int val; * public ListNode next; * public ListNode(int val=0, ListNode next=null) { * this.val = val; * this.next = next; * } * } */ public class Solution { public bool IsPalindrome(ListNode head) { ListNode slow = head; ListNode fast = head.next; while (fast != null && fast.next != null) { slow = slow.next; fast = fast.next.next; } ListNode cur = slow.next; slow.next = null; ListNode pre = null; while (cur != null) { ListNode t = cur.next; cur.next = pre; pre = cur; cur = t; } while (pre != null) { if (pre.val != head.val) { return false; } pre = pre.next; head = head.next; } return true; } }