98题 Sort List

Sort List

Description
Sort a linked list in O(nlogn)O(nlogn) time using constant space complexity.

/**
 * Definition for ListNode
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param head: The head of linked list.
     * @return: You should return the head of the sorted linked list, using constant space complexity.
     */
    public ListNode sortList(ListNode head) {
        // write your code here
        if(head == null || head.next == null){
            return head ;
        }
        ListNode mid = findMid(head) ;
        ListNode right = sortList(mid.next) ;
        mid.next = null ;
        ListNode left = sortList(head) ;
        return merge(left , right) ;
    }
    public ListNode findMid(ListNode head){
        ListNode slow = head , fast = head.next ;
        while(fast != null && fast.next != null){
            fast = fast.next.next ;
            slow = slow.next ;
        }
        return slow ;
    }
    public ListNode merge(ListNode head1 , ListNode head2){
        ListNode dummy = new ListNode(0) ;
        ListNode tail = dummy ;
        while(head1 != null && head2 != null){
            if(head1.val < head2.val){
                tail.next = head1 ;
                head1 = head1.next ;
            }else{
                tail.next = head2 ;
                head2 = head2.next ;
            }
            tail = tail.next ;
        }
        if(head1 != null){
            tail.next = head1 ;
        }else{
            tail.next = head2 ;
        }
        return dummy.next ;
    }
}