Locate a Guaranteed Mountain Peak Logarithmically

### Problem arr is guaranteed to rise strictly to one interior peak and then fall strictly. Return the peak's zero-based index using O(log n) time. ```cpp class Solution { public: int peakIndexInMountainArray(vector<int>& arr); }; ``` ### Examples **Example 1:** ```text arr = [1,4,7,3] Output: 2 Explanation: The increasing side ends at7. ``` **Example 2:** ```text arr = [2,9,8,5,0] Output: 1 Explanation: Every entry after9 lies on the descending side. ``` ### Constraints - Array length lies in [3,100000]. - Entries are in [0,1000000]. - A strict mountain with an interior peak is guaranteed. ### Contract considerations Keep the required logarithmic search. ```hint Read the boundary condition What does the local slope reveal about the peak's side? ``` ### Boundary reminders Keep the required logarithmic search.