Solution.ts

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

function reverseBetween(head: ListNode | null, left: number, right: number): ListNode | null {
    const n = right - left;
    if (n === 0) {
        return head;
    }

    const dummy = new ListNode(0, head);
    let pre = null;
    let cur = dummy;
    for (let i = 0; i < left; i++) {
        pre = cur;
        cur = cur.next;
    }
    const h = pre;
    pre = null;
    for (let i = 0; i <= n; i++) {
        const next = cur.next;
        cur.next = pre;
        pre = cur;
        cur = next;
    }
    h.next.next = cur;
    h.next = pre;
    return dummy.next;
}