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class Solution {
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  public int peakIndexInMountainArray(int[] arr) {
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    int start = 0;
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    int end = arr.length - 1;
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    while (start < end) {
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      int mid = start + (end - start) / 2;
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      // if mid < mid next
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      if (arr[mid] < arr[mid + 1]) {
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        start = mid + 1;
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      }
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      // otherwise it can either peak element or greater element
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      else {
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        end = mid;
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      }
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    }
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    return start; // or we can return end also, bcz both will be on same value at the time, that's
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    // why loop breaks here.
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  }
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}