148. Sort List

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

Example 1:

Input: 4->2->1->3
Output: 1->2->3->4

Example 2:

Input: -1->5->3->4->0
Output: -1->0->3->4->5

divide and conquer + merge sort，先找中点，断开，再merge

time: O(nlogn), space: O(1)

/**
 * 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(head == null || head.next == null) return head;
        ListNode mid = getMiddle(head);
        ListNode second = mid.next;
        mid.next = null;
        return merge(sortList(head), sortList(second));
    }
    
    private ListNode getMiddle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        while(fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
    
    private ListNode merge(ListNode n1, ListNode n2) {
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;
        while(n1 != null && n2 != null) {
            if(n1.val <= n2.val) {
                cur.next = n1;
                n1 = n1.next;
            }
            else {
                cur.next = n2;
                n2 = n2.next;
            }
            cur = cur.next;
        }
        if(n1 != null) cur.next = n1;
        if(n2 != null) cur.next = n2;
        
        return dummy.next;
    }
}