Remove "b" and "ac" from a given string | GeeksforGeeks

Given a string, eliminate all “b” and “ac” in the string, you have to replace them in-place, and you are only allowed to iterate over the string once.

The approach is to use two index variables i and j. We move forward in string using ‘i’ and add characters using index j except ‘b’ and ‘ac’. The trick here is how to track ‘a’ before ‘c’. An interesting approach is to use a two state machine. The state is maintained to TWO when previous character is ‘a’, otherwise state is ONE.
1) If state is ONE, then do NOT copy the current character to output if one of the following conditions is true
…a) Current character is ‘b’ (We need to remove ‘b’)
…b) Current character is ‘a’ (Next character may be ‘c’)
2) If state is TWO and current character is not ‘c’, we first need to make sure that we copy the previous character ‘a’. Then we check the current character, if current character is not ‘b’ and not ‘a’, then we copy it to output.

void stringFilter(char *str)

{

    // state is initially ONE (The previous character is not a)

    int state = ONE;

    // i and j are index variables, i is used to read next character of input

    // string, j is used for indexes of output string (modified input string)

    int j = 0;

    // Process all characters of input string one by one

    for (int i = 0; str[i] != '\0'; i++)

    {

        /* If state is ONE, then do NOT copy the current character to output if

           one of the following conditions is true

           ...a) Current character is 'b' (We need to remove 'b')

           ...b) Current character is 'a' (Next character may be 'c') */

        if (state == ONE && str[i] != 'a' && str[i] != 'b')

        {

            str[j] = str[i];

            j++;

        }

        // If state is TWO and current character is not 'c' (otherwise

        // we ignore both previous and current characters)

        if (state == TWO && str[i] != 'c')

        {

            // First copy the previous 'a'

            str[j] = 'a';

            j++;

            // Then copy the current character if it is not 'a' and 'b'

            if (str[i] != 'a' && str[i] != 'b')

            {

                str[j] = str[i];

                j++;

            }

        }

        // Change state according to current character

        state = (str[i] == 'a')? TWO: ONE;

    }

    // If last character was 'a', copy it to output

    if (state == TWO)

    {

        str[j] = 'a';

        j++;

    }

    // Set the string terminator

    str[j] = '\0';

}

An extension of above problem where we don’t want “ac” in output at all:
The above code looks fine and seems to handle all cases, but what if input string is “aacacc”, the above code produces output as “ac” which looks correct as it removes consecutive occurrences of ‘a’ and ‘c’. What if the requirement is to not have an “ac” in output string at all. Can we modify the above program to produce output as empty string for input “aacacc” and produce output as “d” when input is “abcaaccd”? It turns out that it can also be done with given restrictions. The idea is simple. We need to add following lines inside for loop of the above program.

if (j>1 && str[j-2] == 'a' && str[j-1] =='c')

http://ideone.com/CER6eq
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