[LeetCode] Reverse Linked List II

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} m
 * @param {number} n
 * @return {ListNode}
 */
var reverseBetween = function(head, m, n) {
    var prev = null;
    var now = head;
    var next = head.next;
    var iprev = null;
    var inext = null;
    var ihead = null;
    var itail = null;
    var s = 1;
    
    var newhead = false;
    
    while (now) {
        if (s >= m && s <= n) {
            if (s == m) {
                itail = now;
                iprev = prev;
            }
            if (s == n) {
                ihead = now;
                inext = next;
            }
            now.next = prev;
            prev = now;
            now = next;
            if (now) {
                next = now.next;
            }
            
            if (s == n) {
                if (!iprev) {
                    head = ihead;
                } else {
                    iprev.next = ihead;
                }
                itail.next = inext;
                break;
            }
        } else {
            prev = now;
            now = next;
            if (now) {
                next = now.next;
            }
        }
        s++;
    }
    return head;
};