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import { ListNode } from "../mod.ts";
export default function pairSum(head: ListNode | null): number {    if (!head) return 0;    let left = head;    let right = head;    const node_to_prev = new WeakMap<ListNode, null | ListNode>();
    let sum = 0;
    while (right) {        if (right.next) {            node_to_prev.set(right.next, right);            right = right.next;        } else {            break;        }    }
    while (left !== right) {        sum = Math.max(sum, left.val + right.val);        if (left.next) {            left = left.next;        } else {            break;        }        const prev = node_to_prev.get(right);        if (prev) {            right = prev;        } else {            break;        }    }    return sum;}