#include<stdio.h>
#include<stdlib.h>
#include<string.h>

char* deleteCharacters(char * str, char * charSet)
{
	int hash [256];
	if(NULL == charSet)
		return str;
	for(int i = 0; i < 256; i++)
		hash[i] = 0;
	for(int i = 0; i < strlen(charSet); i++)
		hash[charSet[i]] = 1;
	int currentIndex = 0;
	for(int i = 0; i < strlen(str); i++)
	{
		if(!hash[str[i]])
			str[currentIndex++] = str[i];
	}
	str[currentIndex] = '\0';
	return str;
}

int main()
{
    char s[2] = "a";
    char s2[5] = "aca";
	printf("%s", deleteCharacters(s2, s));
	return 0;
}

Given string a and b. Delete all characters of a that are in b.

Delete Characters.

C