Oops!

char *convert(char *s, int numRows)
{
    /* string length */
    int L = strlen(s);
    /* row numbers */
    int R = numRows;

    /* parameter check */
    if (L == 0 || R == 1)
        return s;

    /* full block numbers */
    /*
     * one 'block' is defined as:
     *     0     
     *     1   5
     *     2 4
     *     3
     */
    int B = L / (2 * R - 2);
    /* remain characters */
    int M = L % (2 * R - 2);
    /* column numbers */
    int C = B * (R - 1) + (M <= R ? 1 : M - R + 1);

    /* allocate R*C 2-dimensional array */
    char **arr = NULL;
    int r, c, b, i;

    arr = calloc(R, sizeof(char *));
    if (!arr)
        return NULL;
    for (r = 0; r < R; r++) {
        arr[r] = calloc(C, sizeof(char));
        if (!arr[r])
            return NULL;
    }

    /* put characters to array one by one */
    i = 0;
    for (b = 0; b < B; b++) {
        /* vertical direction */
        for (r = 0; r < R; r++) {
            arr[r][b * (R - 1)] = s[i++];
        }
        /* diagonal direction */
        for (r = R - 2; r > 0; r--) {
            arr[r][(b + 1) * (R - 1) - r] = s[i++];
        }
    }

    /* remaining characters */
    /* vertical directi n */
    for (r = 0; i < L && r < R; r++) {
        arr[r][B * (R - 1)] = s[i++];
    }
    /* diagonal direction */
    for (r = R - 2; i < L; r--) {
        arr[r][(B + 1) * (R - 1) - r] = s[i++];
    }

    /* reuse input 's' as output */
    i = 0;
    for (r = 0; r < R; r++) {
        for (c = 0; c < C; c++) {
            if (arr[r][c] != '\0')
                s[i++] = arr[r][c];
        }
    }

    /* free 2-dimensional array */
    for (r = 0; r < R; r++)
        free(arr[r]);
    free(arr);

    return s;
}