byte map[32]={0,0……0};

int len=strlen(string);

//PrepareMap
for(int i=0;i<len;i++)
	map[(string[i])>>3] |= 1<<( (string[i]) &7)


//deciding whether a character (ch) is in string
if ( (map[ch>>3] & (1<<(ch&7))) !=0 )
	//ch is in string
else
	//ch is not in string


In the above program, some bit operators are used. Here are their meanings:
op1 >> op2: shift the bits in op1 to the right for op2 bits; neither op1 nor op2 is affected
op1 << op2: shift the bits in op1 to the left for op2 bits; neither op1 nor op2 is affected
op1 & op2: perform bit-wise AND operation between each pair of matching bits
in op1 and op2; neither op1 nor op2 is affected
op1 | op2: perform bit-wise OR operation between each pair of matching bits
in op1 and op2; neither op1 nor op2 is affected