Poison

234. Palindrome Linked List

发表于
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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
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class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head.next == null) {
            return true;
        }

        // d -> 1 -> 2 -> 3
        //                   f
        //           s

        // d -> 1 -> 2 -> 3 -> 4
        //                     f
        //           s

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

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

        return true;
    }

    private ListNode reverse(ListNode node) {
        ListNode pre = null;
        while (node != null) {
            ListNode next = node.next;
            node.next = pre;
            pre = node;
            node = next;
        }

        return pre;
    }
}
Reference

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