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

uniquechars.cpp

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

#define CATCH_CONFIG_MAIN
#include "catch.hpp"
#include <iostream>

using namespace std;

bool find(const char& c, const string& str) {
    for (auto &s:str) if (s == c) return true;
    return false;
}

bool isUnique(string const& str) {
    bool hasMatch = false;
    for (auto i = 0; i < str.size(); i++)
        hasMatch = hasMatch || find(str[i], str.substr(i + 1));
    return !hasMatch;
}

TEST_CASE( "Determine if string has unique characters", "[isUnique]" ) {
    REQUIRE(isUnique("better") == false);
    REQUIRE(isUnique("mountain") == false);
    REQUIRE(isUnique("unique") == false);
    REQUIRE(isUnique("news") == true);
    REQUIRE(isUnique("house") == true);
}