Javascript Program For Removing Middle Points From a Linked List Of Line Segments

Last Updated : 21 Aug, 2022

Given a linked list of coordinates where adjacent points either form a vertical line or a horizontal line. Delete points from the linked list which are in the middle of a horizontal or vertical line.
Examples:

Input:  (0,10)->(1,10)->(5,10)->(7,10)
                                  |
                                (7,5)->(20,5)->(40,5)
Output: Linked List should be changed to following
        (0,10)->(7,10)
                  |
                (7,5)->(40,5) 
The given linked list represents a horizontal line from (0,10) 
to (7, 10) followed by a vertical line from (7, 10) to (7, 5), 
followed by a horizontal line from (7, 5) to (40, 5).

Input:     (2,3)->(4,3)->(6,3)->(10,3)->(12,3)
Output: Linked List should be changed to following
    (2,3)->(12,3) 
There is only one vertical line, so all middle points are removed.

Source: Microsoft Interview Experience

The idea is to keep track of the current node, next node, and next-next node. While the next node is the same as the next-next node, keep deleting the next node. In this complete procedure, we need to keep an eye on the shifting of pointers and checking for NULL values.
Following are implementations of the above idea.

Javascript

<script>
 
// Javascript program to remove middle 
// points in a linked list of
// line segments,
var  head; // head of list
 
    /* Linked list Node */
    class Node {
 
constructor(x , y) {
            this.x = x;
            this.y = y;
            this.next = null;
        }
    }
 
    // This function deletes middle
    // nodes in a sequence of
    // horizontal and vertical line
    // segments represented
    // by linked list.
    function deleteMiddle() {
        // If only one node or no 
        // node...Return back
        if (head == null || head.next == null ||
        head.next.next == null)
            return head;
 
var Next = head.next;
var NextNext = Next.next;
 
        // check if this is vertical or
        // horizontal line
        if (head.x == Next.x) {
            // Find middle nodes with same
            // value as x and
            // delete them.
            while (NextNext != null && 
            Next.x == NextNext.x) 
            {
                head.next = Next.next;
                Next.next = null;
 
                // Update NextNext for the next iteration
                Next = NextNext;
                NextNext = NextNext.next;
            }
        }
 
        // if horizontal
        else if (head.y == Next.y) {
            // find middle nodes with same value as y and
            // delete them
            while (NextNext != null && 
            Next.y == NextNext.y) {
                head.next = Next.next;
                Next.next = null;
 
                // Update NextNext for the next iteration
                Next = NextNext;
                NextNext = NextNext.next;
            }
        }
 
        // Adjacent points should have same x or same y
        else {
            document.write("Given list is not valid");
            return null;
        }
 
        // recur for other segment
 
        // temporarily store the head and move head forward.
var temp = head;
        head = head.next;
 
        // call deleteMiddle() for next segment
        this.deleteMiddle();
 
        // restore head
        head = temp;
 
        // return the head
        return head;
    }
 
    /*
     Given a reference (pointer to pointer) to 
     the head of a list and an int, push
     a new node on the front of the list.
     */
    function push(x , y) {
        /*
         1 & 2: Allocate the Node & Put in the data
         */
var new_node = new Node(x, y);
 
        /* 3. Make next of new Node as head */
        new_node.next = head;
 
        /* 4. Move the head to point to new Node */
        head = new_node;
    }
 
    function printList() {
var temp = head;
        while (temp != null) {
            document.write("(" + temp.x + "," + 
            temp.y + ")->");
            temp = temp.next;
        }
        document.write("<br/>");
    }
 
    /* Driver program to test above functions */
     
 
        push(40, 5);
        push(20, 5);
        push(10, 5);
        push(10, 8);
        push(10, 10);
        push(3, 10);
        push(1, 10);
        push(0, 10);
 
        document.write("Given list<br/>");
        printList();
 
        if (deleteMiddle() != null) {
            document.write("Modified Linked List is<br/>");
            printList();
        }
 
// This code contributed by gauravrajput1
 
</script>

Output:

Given Linked List:
(0,10)-> (1,10)-> (3,10)-> (10,10)-> (10,8)-> (10,5)-> (20,5)-> (40,5)->
Modified Linked List:
(0,10)-> (10,10)-> (10,5)-> (40,5)->

Time Complexity of the above solution is O(n) where n is a number of nodes in the given linked list.

Auxiliary Space: O(n) due to recursive stack space.
Exercise:
The above code is recursive, write an iterative code for the same problem. Please see below for the solution.
Iterative approach for removing middle points in a linked list of line segments
Please refer complete article on Given a linked list of line segments, remove middle points for more details!