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var sortColors = function (arr) {
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  let one = 0,
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    zero = 0,
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    two = 0;
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  // step1
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  for (let elem of arr) {
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    if (elem == 0) zero++;
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    else if (elem == 1) one++;
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    else two++;
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  }
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  // step2
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  arr.length = 0;
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  // step3
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  for (let i = 0; i < zero; i++) arr.push(0);
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  for (let i = 0; i < one; i++) arr.push(1);
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  for (let i = 0; i < two; i++) arr.push(2);
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};