常考数据结构与算法:重排序列

 

public class ReorderListMe {
    public static void main(String[] args) {
        ListNode head = new ListNode(1);
        ListNode n2 = new ListNode(2);
        ListNode n3 = new ListNode(3);
        ListNode n4 = new ListNode(4);
        ListNode n5 = new ListNode(5);
        ListNode n6 = new ListNode(6);

        head.next = n2;
        n2.next = n3;
        n3.next = n4;
        n4.next = n5;
        n5.next = n6;

        ReorderListMe reorderListMe = new ReorderListMe();
        reorderListMe.reorderList(head);

        while(null != head){
            System.out.print(head.val + " ");
            head = head.next;
        }
    }

    public void reorderList(ListNode head) {
        // 从中间位置反转链表的节点
        if(null == head){
            return;
        }

        // 1 2 3 4 5 6
        ListNode slow = head; // 每次走一步
        ListNode fast = head; // 每次走两步
        while(fast.next != null && fast.next.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }

        /*
        开始逆序 , 将链表从中间部分断开，后半部分逆序
        1 2 3
        6 5 4
        */
        ListNode newHead = new ListNode(-1);
        ListNode temp = slow.next;
        slow.next = null;
        slow = temp;
        while(slow != null){
            temp = slow.next;
            slow.next = newHead.next;
            newHead.next = slow;
            slow = temp;
        }

        /*
        * 合并两个链表
        */
        newHead = newHead.next;
        ListNode p = head;
        while(p != null && null !=newHead){
            temp = p.next;
            p.next = newHead;
            p = temp;
            temp = newHead.next;
            newHead.next = p;
            newHead = temp;
        }

    }
}

class ListNode {
    int val;
    ListNode next = null;

    ListNode() {
    }

    ListNode(int val) {
        this.val = val;
    }
}