Check if unique chars

Check if a given string contains unique characters.  Eg: "Hello" contains duplicates, therefore should return false (0).  "Doing" contains unique chars and therefore should return true (1).

/*

 * UniqueChars.cpp

 *

 *  Created on: Apr 10, 2012

 *      Author: Kiran Hegde

 */

#include <string>

#include <iostream>

using namespace std;

/**

 * works with any ASCII char

 */

bool isUniqueChars(string &str) {

    if (str.length() > 256) return false;

    bool charSet[256];

    for (int i=0; i<256; i++) {

        charSet[i] = false;

    }

    for (unsigned int i=0;i<str.length(); i++) {

        int val = str[i];

        if (charSet[val]) {

            return false;

        }

        charSet[val] = true;

    }

    return true;

}

/**

 * Test the function

 */

int main() {

    string words[] = {"hello", "world", "what", "abcdefghijklmnopqrstuvwxyz123456789"};

    for (int i=0; i < 4; i++) {

        cout << "Word: " << words[i] << " Unique? " << isUniqueChars(words[i]) << endl;

    }

    cout << endl;

}