234. Palindrome Linked List

,

Stack

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
class Solution {
    public boolean isPalindrome(ListNode head) {
        // d * * * * *
        // f   f   f   f
        // s s s s

        // d * * * * * *
        // f   f   f   f
        // s s s s

        ListNode dummyHead = new ListNode();
        dummyHead.next = head;

        Stack<Integer> stack = new Stack<>();
        ListNode fast = dummyHead, slow = dummyHead;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast != null) {
                stack.push(slow.val);
            }
        }

        ListNode curr = slow.next;
        while (curr != null && curr.val == stack.peek()) {
            stack.pop();
            curr = curr.next;
        }

        return stack.isEmpty();
    }
}

Linked List

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
class Solution {
    public boolean isPalindrome(ListNode head) {
        // d -> 1
        // f         f
        // s    s

        // d -> 1 -> 2
        // f         f
        // s    s

        // d -> 1 -> 2 -> 2
        // f         f         f
        // s    s    s

        // d -> 1 -> 2 -> 2 -> 1
        // f         f         f
        // s    s    s

        ListNode dummyHead = new ListNode(-1, head);
        ListNode fast = dummyHead, slow = dummyHead;
        while (fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }

        ListNode nodeA = head, nodeB = reverseList(slow.next);
        while (nodeB != null) {
            if (nodeB.val != nodeA.val) {
                return false;
            }
            nodeA = nodeA.next;
            nodeB = nodeB.next;
        }

        return true;
    }

    public ListNode reverseList(ListNode head) {
        ListNode pre = null, curr = head;
        while (curr != null) {
            ListNode next = curr.next;
            curr.next = pre;
            pre = curr;
            curr = next;
        }

        return pre;
    }
}

References

234. Palindrome Linked List
剑指 Offer II 027. 回文链表