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// Restore bracket sequence

function restore(str) {
  let arr = [];
  let counter = 0;
  for(let i=0; i<str.length; i++) {
    if (str[i]=='(') {
      arr.push('(');
      counter++;
    } else { // str[i]==')'
      counter--;
      if (counter < 0) {
        arr.push('(');
      }
      arr.push(')')
    }
  }
  for(let i=0; i<counter; i++) {
    arr.push(')');
  }
  return arr.join('');
}

console.log('===> start');
console.log(restore(')))'));