Fast convert double pointer array into single pointer, probably can be continuous

I am using a library that returns the image as a big 2D array int**. I need to convert it into int* 1D array. I think I've managed to do it quite fast by copying memory blocks:

// have int labels** as 2D array, also have rows and cols 

//create 1D array
int *labels1D = new int[rows*cols];

//copy contents
for(int i = 0; i < rows; i++) {        
    // Here I don't know anything about how those arrays were allocated
    // Maybe std::copy would handle it for me?
    std::copy_n(labels[i], cols, labels1D + i*cols);
}

So the first question is whether I can do something better here? Is everything safe here assuming that library is a black box?

---

I do not want much to modify the library code, but I've found additionally how the source array in my side library this->currentLabels was created:

int** currentLabels; //in class declaration
...
// in the code
this->currentLabels = new int*[this->height];

for (int i = 0; i < this->height; ++i) {
    this->currentLabels[i] = new int[this->width];

    for (int j = 0; j < this->width; ++j) {
     // some code for setting the value
        }

    }

Looks like the values for rows and cols are known.

So the second question is: can I modify this code to make it allocate the 2D array in one memory block:

this->currentLabels = malloc(nrows*sizeof(int*) + (nrows*(ncolumns*sizeof(int)));

to allow me then just map it somehow to my 1D array without copying memory?

int labels1D = labels; // would be an array of [rows*cols]