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# 852. Peak Index in a Mountain Array

![](/files/-MivTjRv2l-0UoSeE74s)

![](/files/-MivTmyfh3Uhjfn5P2vX)

![](/files/-MivTrfWyl2pQb8-F6yl)

time: O(logn)

space: O(1)

```java
class Solution {
    public int peakIndexInMountainArray(int[] arr) {
        int start = 0;
        int end = arr.length - 1;
        
        while (start + 1 < end) {
            int mid = start + (end - start)/2;
            if (arr[mid] < arr[mid+1]) {
                start = mid;
            } else {
                end = mid;
            }
        }
        return Math.max(start, end);
    }
}
```