Javascript Program For Swapping Kth Node From Beginning With Kth Node From End In A Linked List

// A javascript program to swap kth
// node from the beginning with
// kth node from the end
class Node {
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}

let head;

/* Utility function to insert a node 
   at the beginning */
function push(new_data) {
    new_node = new Node(new_data);
    new_node.next = head;
    head = new_node;
}

/* Utility function for displaying 
   linked list */
function printList() {
    node = head;
    while (node != null) {
        console.log(node.data + " ");
        node = node.next;
    }
    console.log("");
}

/* Utility function for calculating length 
   of linked list */
function countNodes() {
    let count = 0;
    s = head;
    while (s != null) {
        count++;
        s = s.next;
    }
    return count;
}

/* Function for swapping kth nodes from 
   both ends of linked list */
function swapKth(k) {
    // Count nodes in linked list
    let n = countNodes();

    // Check if k is valid
    if (n < k)
        return;

    // If x (kth node from start) and
    // y(kth node from end) are same
    if (2 * k - 1 == n)
        return;

    // Find the kth node from beginning 
    // of linked list. We also find previous 
    // of kth node because we need to update 
    // next pointer of the previous.
    x = head;
    x_prev = null;
    for (i = 1; i < k; i++) {
        x_prev = x;
        x = x.next;
    }

    // Similarly, find the kth node from end 
    // and its previous. kth node from end 
    // is (n-k+1)th node from beginning
    y = head;
    y_prev = null;
    for (i = 1; i < n - k + 1; i++) {
        y_prev = y;
        y = y.next;
    }

    // If x_prev exists, then new next of it 
    // will be y. Consider the case when y->next 
    // is x, in this case, x_prev and y are same. 
    // So the statement "x_prev->next = y" creates 
    // a self loop. This self loop will be broken 
    // when we change y->next.
    if (x_prev != null)
        x_prev.next = y;

    // Same thing applies to y_prev
    if (y_prev != null)
        y_prev.next = x;

    // Swap next pointers of x and y. These 
    // statements also break self loop if 
    // x->next is y or y->next is x
    temp = x.next;
    x.next = y.next;
    y.next = temp;

    // Change head pointers when k is 1 or n
    if (k == 1)
        head = y;

    if (k == n)
        head = x;
}

// Driver code
for (let i = 8; i >= 1; i--)
    push(i);

console.log("Original linked list:");
printList();

for (let i = 1; i < 9; i++) {
    swapKth(i);
    console.log("Modified List for k = " + i);
    printList();
}
// This code is contributed by gauravrajput1