92-Reverse-Linked-List-II.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} left
 * @param {number} right
 * @return {ListNode}
 */
var reverseBetween = function (head, left, right) {
    if (!head || !head.next || left === right) {
        return head;
    }
    let idx = 1;
    let prevLower = null;
    let curLower = head;
    while (curLower && idx < left) {
        prevLower = curLower;
        curLower = curLower.next;
        idx++;
    }
    let storedPrevLower = prevLower;
    let storedCurLower = curLower;
    let prevPtr = null;
    let tmpPtr = null;
    while (curLower && idx <= right) {
        tmpPtr = curLower.next;
        curLower.next = prevPtr;
        prevPtr = curLower;
        curLower = tmpPtr;
        idx++;
    }
    if (storedPrevLower) {
        storedPrevLower.next = prevPtr;
    }
    storedCurLower.next = curLower;
    return storedPrevLower ? head : prevPtr;
};