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export default function isValid(s: string): boolean {    if (s.length === 0) return true;    if (s.length % 2) return false;    const stk = [];    for (const ch of s) {        if (pairs.has(ch)) {            if (stk.length === 0) return false;            if (stk.slice(-1)[0] !== pairs.get(ch)) return false;            stk.pop();        } else {            stk.push(ch);        }    }    return stk.length === 0;}const pairs = new Map([    [")", "("],    ["]", "["],    ["}", "{"],]);