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 partition(head: ListNode | null, x: number): ListNode | null {
    const [left, right] = [new ListNode(), new ListNode()];
    let [p1, p2] = [left, right];
    for (; head; head = head.next) {
        if (head.val < x) {
            p1.next = head;
            p1 = p1.next;
        } else {
            p2.next = head;
            p2 = p2.next;
        }
    }
    p1.next = right.next;
    p2.next = null;
    return left.next;
}