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Determine if a string has all unique characters (c++)

7/14/2015

 
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/******************************************************************************************
*******************************************************************************************
Chapter 1 Arrays and Strings

 * Implement an algorithm to determine if a string has all unique characters.
 * What if you cannot use additional data structures?

By: Hamed Kiani (July 14, 2015)
******************************************************************************************
******************************************************************************************/

#include<iostream>

using namespace std;

// method 1:
// using bit shifting, time: O(n), space: O(1)
// only for string in [a-z], 28 chars
// 1 (int) is a 4 bytes (32 bits) to count the presence of 28 chars
// challenge: how to re-write this function to work for all 256 ASCII chars??

bool string_unique_bit(char *str)
{
        int check = 0;
        int i = 0;    // counter over the string
        // '/0' indicates the end of a c-string
        while(str[i] != '\0')
        {
                // (int) str[i]: cast the char to int; (int)'a' = 97
                int pos = (int) str[i] - 'a';
                // (1 << pos) shifts the bit representation of 1 (32 bits) to left by pos-steps
                // if ((check & (1 << pos)) >0 ) means that current char was seen before
                if ((check & (1 << pos)) >0 )                     
                        return false;
                // keeping the presence of each char in check
                check |= (1 << pos);
                // process the next char
                i++;
        }
        return true;
}


// method 2:
// using an aditional buffer to keep the history of each seen char
// time: O(n), space: O(n)
// works for all ASCII chars: 0-255
// challenge: how to write this function without using i?

bool string_unique(char * str)
{
        // check is a bool array of 256, initialized as false      
        bool check[256] = {false};
        int i = 0;
        while(str[i] != '\0')
        {
                if (check[(int) str[i]])
                        // current char was seen before, 
                        return false;
                else
                {
                        check[(int) str[i]] = true;
                        i++;
                }
        }
}

// Test it!
int main()
{
        char * str;
        str = "hamed";
        bool check;
        check = string_unique(str);
        if (check)
                cout << str << " has all unique chars"  << endl;
        else
                cout << str << " does not have unique chars"  << endl;   
        return 0;
}

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