Update the character
if ( ( int ) ( S [ i ] ) + add <= ( int ) ( ' z ' ) ) S [ i ] = ( char ) ( ( int ) ( S [ i ] ) + add ) ; else { add = ( ( int ) ( S [ i ] ) + add ) - ( ( int ) ( ' z ' ) ) ; S [ i ] = ( char ) ( ( int ) ( ' a ' ) + add - 1 ) ; } }