Poison

143. Reorder List

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class Solution {
    public void reorderList(ListNode head) {
        // d -> 1 -> 2 -> 3 -> 4
        //                     f
        //           s

        // d -> 1 -> 2 -> 3 -> 4 -> 5
        //                              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 nodeB = reversedRightHead, nodeA = head;
        while (nodeB != null) {
            ListNode nextA = nodeA.next;
            nodeA.next = nodeB;
            ListNode nextB = nodeB.next;
            nodeB.next = nextA;
            nodeA = nextA;
            nodeB = nextB;
        }
    }

    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

143. Reorder List
剑指 Offer II 026. 重排链表