Remove duplicates in a string.

Solution #1: Let us first consider the case when extra buffer is not allowed. Scan every character in the string and compare it with every previous character, if it is a duplicate then ignore it, otherwise copy it. This approach will have a time complexity of O(n^2). Here is the code,

void removeDuplicates(char* ptr)
{
    int end = 1;
    int length = strlen(ptr);
    int current;
    int i;

    current = 1;

    for(end=1; end<length; end++)
    {
        for(i=0; i<end; i++)
        {
            if(ptr[end] == ptr[i])
                break;
        }

        if(i == end)
        {
            ptr[current] = ptr[i];
            current++;
        }
    }
    ptr[current] = 0;
}

int main(int argc, char** argv)
{
    char str[256] = {0,};

        gets(str);

    removeDuplicates(str);

    printf("%s\n", str);

    return 1;
}

2) Now, let's consider the case when we have the luxury of using extra memory. Also consider that the string is made up of extended ASCII characters. So let's make a bit array of 256 bits. Now, we can scan the string and use this array to keep a track of all the characters we encounter and ignore the duplicate ones.