6ms Quick Sort Solution in Java


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    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        }
        
        ListNode mid = findMedian(head);
        
        ListNode leftDummy = new ListNode(0), leftTail = leftDummy;
        ListNode rightDummy = new ListNode(0), rightTail = rightDummy;
        ListNode middleDummy = new ListNode(0), middleTail = middleDummy;
        while (head != null) {
            if (head.val < mid.val) {
                leftTail.next = head;
                leftTail = head;
            } else if (head.val > mid.val) {
                rightTail.next = head;
                rightTail = head;
            } else {
                middleTail.next = head;
                middleTail = head;
            }
            head = head.next;
        }
        
        leftTail.next = null;
        middleTail.next = null;
        rightTail.next = null;
        
        ListNode left = sortList(leftDummy.next);
        ListNode right = sortList(rightDummy.next);
        
        return concat(left, middleDummy.next, right);
    }
    
    private ListNode findMedian(ListNode head) {
        ListNode slow = head, fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
    
    private ListNode concat(ListNode left, ListNode middle, ListNode right) {
        ListNode dummy = new ListNode(0), tail = dummy;
        
        tail.next = left; tail = getTail(tail);
        tail.next = middle; tail = getTail(tail);
        tail.next = right; 
        return dummy.next;
    }
    
    private ListNode getTail(ListNode head) {
        if (head == null) {
           return null;
        } 
       
        while (head.next != null) {
            head = head.next;
        }
        return head;
    }

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