Checking for uniqueness within a string

I saw this interview question in the book Cracking the Coding Interview:


Implement an algorithm to determine if a string has all unique characters

The authors solution uses bit-shifting for a pretty efficient solution.

I wanted to give it a shot to discover some new things and practice some. I'd like for this to be a discussion of how I can improve my programming and also, what the correct answer may be. Did I leave out any edge-cases? Is there a more efficient way of doing this?

#include 
#include 
#include 

using namespace std;

// Uses an iterator to display the map.
void displayMap(map displayCharacters)
{
    for (map::iterator itr = displayCharacters.begin(), end = displayCharacters.end(); itr != end; ++itr)
    {
        cout first  " second  uniqueCharacters)
{
    for (int i = 0; i  readString(string stringRead, map uniqueCharacters)
{
    for (int i = 0, strLen = stringRead.length(); i  uniqueCharacters;
    string testString = "aaabbbccc";
    int stringLength = testString.length();

    // uniqueCharacters is mapped to the above string and the displayed using displayMap.
    uniqueCharacters = readString(testString, uniqueCharacters);
    displayMap(uniqueCharacters);

    if (isUnique(stringLength, testString, uniqueCharacters))
    {
        cout << "Unique string!\n";
    }
    else
    {
        cout << "Not unique!\n";
    }

    cin.ignore();
    cin.get();
}

This solution is inefficient:

• There is no point counting the characters in the entire string: once you found a duplicate, you can stop counting, because you know the result is false

• This also implies that the map of counts is wasted space

• Iterating over the alphabet to check character counts is inefficient: it would be better to iterate over the map values

Instead of a map, it would be more efficient to use a set.

Instead of a set, you could also consider a boolean array of the size of the alphabet, values initialized to false,
and use that to track characters seen so far.
The disadvantage of this approach over a set is that it might use more space.

You are also violating some good practices:

• using namespace std is considered bad practice

• readString is poorly named: it doesn't "read a string", it initializes a map of character counts to all zero values

• No need to pass stringLength together with testString. You can derive that value from testString

StackExchange Code Review Q#98049, answer score: 3