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var peakIndexInMountainArray = function (arr) {
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  //lets assume we have peak it divides array in two parts
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  // first part is increasing order , second part is decreasing
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  // when we find the middle we'll compare arr[middle] > arr[middle+1], it means
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  //we can only find max in first part of arr (increasing part) else second part.
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  //there will be point where start === end that is our peak
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  let start = 0;
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  let end = arr.length - 1;
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  while (start < end) {
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    let mid = parseInt(start + (end - start) / 2);
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    if (arr[mid] > arr[mid + 1]) {
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      end = mid;
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    } else {
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      start = mid + 1;
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    }
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  }
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  return start;
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};