#include <iostream>
#include <math.h>
 
int concat(int* arr, size_t len);
 
int main()
{
    const size_t len = 3;
    int arr[len] = {12, 345, 552};
    std::cout << concat(arr, len) << std::endl;
 
    return 0;
}
 
int concat(int* arr, size_t len)
{
    int result = 0;
 
    for (int i = 0; i < len; i++)
    {
        int digits = floor(log10(arr[i])) + 1;
        result *= pow(10, digits);
        result += arr[i];
    }
 
    return result;
}

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