Leetcode148-Sort_List

Sort_List

在LeetCode 里面，因为只有归并排序的时间复杂度为O(1)，所以快速排序用不了，前面两个都没用直接看最后一个归并排序。

冒泡排序（超时了)

public ListNode sortList(ListNode head) {
        
        if(null == head)
            return head;
        
        int counter = 0;
        ListNode current = head;
        while (null != current.next) {
            current = current.next;
            counter++;
        }

        current = head;
        int pre;
        while (counter > 0) {
            for (int i = 0; i < counter; i++) {
                if (current.val > current.next.val) {
                    pre = current.val;
                    current.val = current.next.val;
                    current.next.val = pre;
                }
                current = current.next;
            }
            current = head;
            counter--;
        }
        return head;
        
    }

　　

快速排序（在leetcode 上面还是超时间了）

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {        
        quickSort(head, null);
        return head;
        
    }
    public static ListNode quickSort(ListNode head, ListNode end) {
        if (head != end) {

            ListNode p1 = head;
            ListNode p2 = head.next;

            //走到末尾才停
            while (p2 != null) {

                //大于key值时，p1向前走一步，交换p1与p2的值
                if ((p2.val < head.val) && (p1 != p2)) {
                    p1 = p1.next;
                    int temp = p1.val;
                    p1.val = p2.val;
                    p2.val = temp;
                }
                p2 = p2.next;
            }

            //当有序时，不交换p1和key值
            if (p1 != head) {
                int temp = p1.val;
                p1.val = head.val;
                head.val = temp;
            }
            quickSort(head, p1);
            quickSort(p1.next, null);
            return p1;

        }
        return head;
    }
}

　　

归并排序

思路：https://blog.csdn.net/mine_song/article/details/69831827 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        
         if (null == head || null == head.next) {
            return head;
        }
        ListNode mid = getMidNode(head);
        ListNode right = mid.next;
        mid.next = null;
        return mergeSort(sortList(head), sortList(right));
        
    }

    static ListNode getMidNode(ListNode node) {

        ListNode fast = node;
        ListNode slow = node;

        while (null != fast.next && null != fast.next.next) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }

 static ListNode mergeSort(ListNode head1, ListNode head2) {

        ListNode p1 = head1;
        ListNode p2 = head2;
        ListNode head;
        if (p1.val < p2.val) {
            head = p1;
            p1 = p1.next;
        } else {
            head = p2;
            p2 = p2.next;
        }
        ListNode p = head;
        while (null != p1 && null != p2) {
            if (p1.val <= p2.val) {
                p.next = p1;
                p1 = p1.next;
                p = p.next;
            } else {
                p.next = p2;
                p2 = p2.next;
                p = p.next;
            }
        }

        if (null != p1) {
            p.next = p1;
        }
        if (null != p2) {
            p.next = p2;
        }
        return head;
    }
}