leetcode148. Sort List

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本文介绍了一种在O(nlogn)时间内使用常数空间复杂度对链表进行排序的方法。采用归并排序策略，利用快慢指针寻找中点，并递归地将链表分为两部分进行排序。

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148. Sort List

Sort a linked list in O(n log n) time using constant space complexity.

解法

归并排序。用快慢指针法找到中间结点。

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

        ListNode mid = findMiddle(head);

        ListNode right = sortList(mid.next);
        mid.next = null;
        ListNode left = sortList(head);


        return merge(left, right);
    }
    private ListNode findMiddle (ListNode head) {
        ListNode slow = head;
        ListNode fast = head.next;
        while (fast!= null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    private ListNode merge(ListNode left, ListNode right) {
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;

        while (left != null && right != null) {
            if (left.val < right.val) {
                head.next = left;
                left = left.next;
            } else {
                head.next = right;
                right = right.next;
            }
            head = head.next;
        }
        if (left != null) {
            head.next = left;
        } 
        if (right != null) {
            head.next = right;
        }
        return dummy.next;
    }
}