Referência para TOANDFRO (Luiza)

#include <stdio.h>

char dec[210][25];
char mess[210];
int n;

int main() {
    while (1) {
        scanf("%d", &n);
        if (!n) break;

        scanf(" %s", mess);
        int line=0; int cnt=0;

        for (int j=0; mess[j]!='\0'; j++) {
            if (line%2) dec[line][n-j%n-1]=mess[j];
            else dec[line][j%n]=mess[j];
            cnt++;

            if (cnt==n) cnt=0, line++;
        }

        for (int j=0; j<n; j++)
            for (int i=0; i<line; i++)
                printf("%c", dec[i][j]);
        printf("\n");
    }

    return 0;
}