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/**
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 * @param {number[]} arr
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 * @return {number[]}
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 */
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var threeEqualParts = function (arr) {
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  const ones = arr.reduce((s, n) => s + n, 0);
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  if (ones === 0) return [0, 2];
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  if (ones % 3 !== 0) return [-1, -1];
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  let onesToFind = ones / 3;
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  let k = arr.length;
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  while (onesToFind > 0) if (arr[--k] === 1) --onesToFind;
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  const iter = arr.length - k;
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  const firstOne = arr.indexOf(1);
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  const secondOne = arr.indexOf(1, firstOne + iter);
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  for (let i = 0; i < iter; i++)
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    if (arr[i + firstOne] !== arr[k + i] || arr[i + secondOne] !== arr[k + i])
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      return [-1, -1];
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  return [firstOne + iter - 1, secondOne + iter];
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};