852. Peak Index in a Mountain Array.cpp

int peakIndexInMountainArray(vector<int>& arr){
    int start = 0, end = arr.size() - 1;
    int mid = start + (end - start) / 2;
    
    while(start < end){
        if(arr[mid] < arr[mid+1])
            start = mid + 1;
        else end = mid;
        mid = start + (end - start)/2;
    }
    return start;
}