H-Index II — C++

// Time:  O(logn)
// Space: O(1)

class Solution {
public:
    int hIndex(vector<int>& citations) {
        const int n = citations.size();
        int left = 0;
        int right = n - 1;
        while (left <= right) {
            const auto mid = left + (right - left) / 2;
            if (citations[mid] >= n - mid) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return n - left;
    }
};