Leetcode 148. Sort List

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

---

Analysis:

1. use merge sort

2. use quick sort

---

Java code

20160601 

merge sort:

/**
 * 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) {
        //use merge sort
        //base case
        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) {
        //base case
        if (head == null || head.next == null) {
            return head;
        }
        ListNode slow = head, fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    
    private ListNode merge(ListNode left, ListNode right) {
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        while (left != null && right != null) {
            if (left.val <= right.val) {
                cur.next = left;
                left = left.next;
            } else {
                cur.next = right;
                right = right.next;
            }
            cur = cur.next;
        }
        if (left != null) {
            cur.next = left;
        }
        if (right != null) {
            cur.next = right;
        }
        return dummy.next;
    }
}