asdf sdfg
asdf ghjk

asdfg
asdfghjk

#include <cstdio>
#include <cstring>
int const MAX = 1e5 + 5;
int next[MAX];
char s1[MAX], s2[MAX];

void get_next(char *a)
{
    int i = 0, j = -1;
    next[0] = -1;
    while(a[i] != '\0')
    {
        if(j == -1 || a[i] == a[j])
        {
            i++;
            j++;
            if(a[i] == a[j])
                next[i] = next[j];
            else
                next[i] = j;
        }
        else
             j = next[j];
    }
}

int KMP(char *a, char *b)
{
    get_next(b);
    int i = 0, j = 0;
    while(a[i] != '\0')
    {
        if(j == -1 || a[i] == b[j])
        {
            i++;
            j++;
        }
        else
            j = next[j];
    }
    return j;
}

int main()
{
    while(scanf("%s %s", s1, s2) != EOF)
    {
        int l1 = KMP(s1, s2);
        int l2 = KMP(s2, s1);
        if(l1 == l2)
        {
            if(strcmp(s1, s2) < 0)
                printf("%s%s\n", s1, s2 + l1);
            else
                printf("%s%s\n", s2, s1 + l1);
        }
        else if(l1 < l2)
            printf("%s%s\n", s2, s1 + l2);
        else
            printf("%s%s\n", s1, s2 + l1);
    }
}