4.c

#include <assert.h>
#include <stdlib.h>
#include <stdio.h>

int lg2(int a, int b)
{
    char af[256];
    char bf[256];
    snprintf(af, 256, "%d%d", a, b);
    snprintf(bf, 256, "%d%d", b, a);
    if (atoi(af) > atoi(bf))
        return atoi(af);
    else
        return atoi(bf);
}

int largest_arrangement(int *numbers, int sz)
{
    assert(sz >= 0);
    if (sz == 1) {
        return numbers[0];
    } else if (sz == 2) {
        return lg2(numbers[0], numbers[1]);
    }

    int split = sz / 2;
    int a = largest_arrangement(numbers, split);
    int b = largest_arrangement(numbers + split, sz - split);
    return lg2(a, b);
}

int main(int argc, char **argv)
{
    int arraysz = argc - 1;
    int numbers[arraysz];

    for (int i = 0; i < arraysz; i++) {
        numbers[i] = atoi(argv[i + 1]);
    }

    int n = largest_arrangement(numbers, arraysz);

    printf("%d\n", n);

    return EXIT_SUCCESS;
}