#include <vector>

class Solution {
public:
    int peakIndexInMountainArray(const std::vector<int>& data) {
        int left_idx = 1;
        int right_idx = data.size() - 2;
        int middle = left_idx + (right_idx - left_idx) / 2;
        while (left_idx < right_idx) {
            if (data[middle] < data[middle + 1]) {
                left_idx = middle + 1;
            } else {
                right_idx = middle;
            }
            middle = left_idx + (right_idx - left_idx) / 2;
        }
        return left_idx;
    }
};


// logarithmic time, constant memory