Java单链表归并排序

概念

归并排序是建立在归并操作上的一种有效的排序算法。该算法是采用分治法（Divide and Conquer）的一个非常典型的应用，归并排序将两个已排序的表合并成一个表。

归并排序基本原理

通过对若干个有序结点序列的归并来实现排序。
所谓归并是指将若干个已排好序的部分合并成一个有序的部分。

单链表实现归并排序

找到中间点拆分链表

//找到中间点，然后分割    public ListNode getMiddle(ListNode head) {        if (head == null) {            return head;        }        //快慢指针        ListNode slow, fast;        slow = fast = head;        while (fast.next != null && fast.next.next != null) {            slow = slow.next;            fast = fast.next.next;        }        return slow;    }

合并排好序的链表

// 合并排好序的链表    public ListNode merge(ListNode a, ListNode b) {        ListNode dummyHead, curr;        dummyHead = new ListNode(0);        curr = dummyHead;        while (a != null && b != null) {            if (a.val <= b.val) {                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;    }

单链表的归并排序

//单链表的归并排序    public ListNode merge_sort(ListNode head) {        if (head == null || head.next == null) {            return head;        }        //得到链表中间的数        ListNode middle = getMiddle(head);        ListNode sHalf = middle.next;        //拆分链表        middle.next = null;        //递归调用        return merge(merge_sort(head), merge_sort(sHalf));    }

Application.java

public static void main(String[] args) {        ListNode head = new ListNode(0);        ListNode l1 = new ListNode(2);        ListNode l2 = new ListNode(5);        ListNode l3 = new ListNode(3);        ListNode l4 = new ListNode(8);        ListNode l5 = new ListNode(4);        ListNode l6 = new ListNode(2);        ListNode l7 = new ListNode(1);        head.next = l1;        l1.next = l2;        l2.next = l3;        l3.next = l4;        l4.next = l5;        l5.next = l6;        l6.next = l7;        l7.next = null;        ListNode p = head;        while (p.next != null) {            System.out.print(p.val);            p = p.next;        }        System.out.print(p.val);        System.out.println();        new MergeSort().merge_sort(head);        p = head;        while (p != null) {            System.out.print(p.val);            p = p.next;        }    }

执行效果图