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var validMountainArray = function (arr) {
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  let index = 0,
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    length = arr.length;
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  //find the peak
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  while (index < length && arr[index] < arr[index + 1]) index++;
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  //edge cases
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  if (index === 0 || index === length - 1) return false;
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  //check if starting from peak to end of arr is descending order
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  while (index < length && arr[index] > arr[index + 1]) index++;
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  return index === length - 1;
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};