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var isNumber = function (S) {
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  let exp = false,
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    sign = false,
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    num = false,
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    dec = false;
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  for (let c of S)
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    if (c >= "0" && c <= "9") num = true;
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    else if (c === "e" || c === "E")
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      if (exp || !num) return false;
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      else (exp = true), (sign = false), (num = false), (dec = false);
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    else if (c === "+" || c === "-")
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      if (sign || num || dec) return false;
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      else sign = true;
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    else if (c === ".")
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      if (dec || exp) return false;
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      else dec = true;
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    else return false;
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  return num;
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};