Algs4-2.2.17链表归并排序

Description

归并排序是链表排序的最佳方法，因为它不需要额外的空间，且运行时间是线性对数级别的。
这里的代码是从网页：http://www.dontforgettothink.com/2011/11/23/merge-sort-of-linked-list/
拷贝来的。这里要谢谢原作者

Code

private class Node {
        Comparable info;
        Node next;
    }

    Node head;
    Node tail;

    public Node merge_sort(Node head){
        if (head == null || head.next == null) {
            return head;
        }

        Node middle = getMiddle(head); //get the middle of the list
        Node sHalf = middle.next; middle.next = null; //split the list into two halfs

        return merge(merge_sort(head),merge_sort(sHalf)); //recurse on that;
    }

    public Node merge(Node a, Node b){
        Node dummyHead, curr; dummyHead = new Node(); curr = dummyHead;
        while (a!=null && b!=null){
            if (!less(b.info, a.info)) {curr.next = a; a = a.next;}
            else { curr.next = b; b = b.next;}
            curr=curr.next;
        }
        curr.next = (a == null) ? b : a;
        return dummyHead.next;
    }

    public Node getMiddle(Node head){
        if (head == null) {return head;}
        Node slow, fast; slow = fast = head;
        while(fast.next != null && fast.next.next != null){
            slow = slow.next; fast = fast.next.next;
        }

        return slow;
    }


    private static boolean less(Comparable v, Comparable w){
        return v.compareTo(w) < 0;
    }

Ref:

• http://www.dontforgettothink.com/2011/11/23/merge-sort-of-linked-list/