String Reverse

 

Question:

    Implement strrev is a possible interview question. How would you do it ?

The simple way is to go to the back of the string, allocate a buffer and push the character from the right to left into the buffer and return it.

But the above approach use extra space

Code:

char* str_rev(char* str)

{

    char* istr = str;

    char* output= (char*) malloc(strlen(str));

    char* ioutput = output;

    while(*str != '\0') {

        str++;

    }

    while(--str != istr) {

        *output = *str;

        output++;

    }

    *output = *str;

    *(output+1) = '\0';

    return ioutput;

}

void main()

{

    char* rstr = str_rev("rajan");

}

 

How would you do without using this extra space?

Just swap the last element in the string with the first element until you reach the middle.

char* str_rev(char* str)

{

    char* tstr = str;

    char* istr = tstr; // string start

    while(*tstr != '\0') tstr++;

    char* estr = --tstr; // string end

    while(istr != estr)

    {

        char tmp = *istr;

        *istr = *estr;

        *estr = tmp;

        istr++;

        estr--;

    }

    return str;

}

void main()

{

    char* ptr = strdup("rajan");

    char* rstr = str_rev(ptr);

}

Important points:

• if char* ptr=”rajan” is used, the above program will crash since, “rajan” is a const literal present part of code segment which cannot be modified.
• strdup creates a new pointer from the input const char*. We use that to remove the const
• free on ptr should be called !!