if (matrix.empty() || matrix[0].empty()) return false;
int n = matrix.size(), m = matrix[0].size();
int l = 0, r = n * m - 1;
        while (l < r)
        {
            int mid = (l + r) / 2;
            if (matrix[mid / m][mid % m] >= target) r = mid;
            else l = mid + 1;
        }
        return matrix[r / m][r % m] == target;

    mid / m 行
    mid % m 列