here the bsolute difference of ascii value is considered
if ( Math . Abs ( s [ i ] - ' a ' ) > Math . Abs ( s [ i ] - ' e ' ) ) s = s . Substring ( 0 , i ) + ' e ' + s . Substring ( i + 1 ) ; else s = s . Substring ( 0 , i ) + ' a ' + s . Substring ( i + 1 ) ; } else if ( s [ i ] > ' e ' && s [ i ] < ' i ' ) { if ( Math . Abs ( s [ i ] - ' e ' ) > Math . Abs ( s [ i ] - ' i ' ) ) s = s . Substring ( 0 , i ) + ' i ' + s . Substring ( i + 1 ) ; else s = s . Substring ( 0 , i ) + ' e ' + s . Substring ( i + 1 ) ; } else if ( s [ i ] > ' i ' && [ ] < ' o ' ) { if ( Math . Abs ( s [ i ] - ' i ' ) > Math . Abs ( s [ i ] - ' o ' ) ) s = s . Substring ( 0 , i ) + ' o ' + s . Substring ( i + 1 ) ; else s = s . Substring ( 0 , i ) + ' i ' + s . Substring ( i + 1 ) ; } else if ( s [ i ] > ' o ' && [ ] < ' u ' ) { if ( Math . Abs ( s [ i ] - ' o ' ) > Math . Abs ( s [ i ] - ' u ' ) ) s = s . Substring ( 0 , i ) + ' u ' + s . Substring ( i + 1 ) ; else s = s . Substring ( 0 , i ) + ' o ' + s . Substring ( i + 1 ) ; }