Code for changed question :
{assign var = 'lastFoundLetter' value= ''}
{foreach from=$rootCategories item=category}
{assign var = 'curFirstLetter' value= {$category.name|substr:0:1|upper}}
{if $curFirstLetter neq $lastFoundLetter}
{if $lastFoundLetter neq ''}<br />{/if}
{$curFirstLetter}
{assign var = 'lastFoundLetter' value=$curFirstLetter}
{/if}
{$category.name}
{/foreach}