无序链表的归并排序 - Java代码纯享版

public class ListNodeMergeSort {

    public static class ListNode{
        int val;
        ListNode next;
        public ListNode(int val){
            this.val = val;
        }
        public ListNode(){}

        public void setVal(int x){
            this.val = x;
        }

        public int getVal(){
            return this.val;
        }
    }

    public ListNode merge(ListNode list1, ListNode list2){
        ListNode dummyList = new ListNode(-1);

        // 临时变量
        ListNode tmp = dummyList;
        ListNode tmp1 = list1;
        ListNode tmp2 = list2;

        // 开始有序合并
        while (tmp1!=null && tmp2!=null){
            if(tmp1.val<tmp2.val){
                tmp.next = tmp1;
                tmp1 = tmp1.next;
            }else{
                tmp.next = tmp2;
                tmp2 = tmp2.next;
            }
            tmp = tmp.next;
        }

        // 剩余链表合并
        if (tmp1!=null){
            tmp.next = tmp1;
        }else if(tmp2!=null){
            tmp.next = tmp2;
        }

        return dummyList.next;
    }

    public ListNode sortList(ListNode head, ListNode tail){
        // 节点为空
        if(head == null){
            return head;
        }

        // 只有一个节点
        if(head.next == tail){
            head.next = null;
            return head;
        }

        // 开始找到中点
        ListNode fast = head;
        ListNode slow = head;
        while (fast!=tail){
            fast = fast.next;
            slow = slow.next;
            if(fast!=null){
                fast = fast.next;
            }
        }

        // 划分为head->mid-1 mid->tail
        ListNode mid = slow;
        ListNode list1 = sortList(head,mid);    //mid为下一个链表的开始
        ListNode list2 = sortList(mid, tail);
        ListNode sorted = merge(list1, list2);  //合并两个有序的链表
        return sorted;
    }

    public ListNode sortList(ListNode head){
        return sortList(head, null);
    }

    public static void main(String[] args) {
        ListNode head = new ListNode(5);
        ListNode cur = head;
        for(int i=5;i>=0;i--){
            cur.next = new ListNode(i);
            cur = cur.next;
        }

        // 排序之前
        ListNode tmp = head;
        System.out.println("排序之前");
        while (tmp!=null){
            System.out.printf(tmp.val+ " ");
            tmp = tmp.next;
        }
        System.out.println();

        // 排序之后
        ListNodeMergeSort listNodeMergeSort = new ListNodeMergeSort();
        head = listNodeMergeSort.sortList(head);
        tmp = head;
        System.out.println("排序之后");
        while (tmp!=null){
            System.out.printf(tmp.val+ " ");
            tmp = tmp.next;
        }
        System.out.println();
    }

}

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