Single LL Operations

single_linked_list.js

/**
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */

function length(head) {
    if (head) {
        let count = 1;
        let tail = head;

        while (tail.next) {
            tail = tail.next;
            count += 1;
        }
        return [tail, count];
    }
    return [head, 0];
}

function middle(head) {
    let slow = head;
    let fast = head;

    while (fast && fast.next) {
        slow = slow.next;
        fast = fast.next.next;
    }

    return slow;
}

function reverse(head) {
    let prev = null;
    let curr = head;
    let next = head.next;

    while (curr) {
        curr.next = prev;
        prev = curr;
        curr = next;
        next = next && next.next;
    }
    
    return prev;
}

function merge(list1, list2) {
    const head = new ListNode();
    
    let node = head;
    while (list1 && list2) {
        if (list1.val < list2.val) {
            node.next = list1;
            list1 = list1.next;
        } else {
            node.next = list2;
            list2 = list2.next;
        }
        node = node.next;
    }
    node.next = list1 || list2;
    
    return head.next;
}

function rotateRight(head, k) {
    if (head && head.next) {
        const [tail, len] = length(head);
        const rotations = k % len;
        const skip = len - rotations;

        tail.next = head;
        let node = head;
        for (let i = 0; i < skip - 1; i++) {
            node = node.next;
        }
        head = node.next;
        node.next = null;
    }
    return head;
}