Different ways to check for duplicates in a string

gistfile1.c

#include <iostream>
#include <string>
#include <map>
#include <algorithm>
#include <climits>

// using STL MAP
bool duplicate_check_maphist(std::string s) 
{
	std::map<char, int> h;
	for (int i = 0; i < s.length(); ++i) 
		if (h[s[i]] == 1) return true;
		else h[s[i]] += 1;
	return false;
}

// using an array to store histogram
bool duplicate_check_arrayhist(std::string s)
{
	int h[255];
	for(int i=0; i<255; h[i]=0, ++i);
	for(int i=0; i<s.length(); ++i) 
		if (h[s[i]] == 1) return true;
		else h[s[i]] += 1;
	return false;
}

// check every pair of characters (O(n^2))
bool duplicate_check_inplace(std::string s)
{
	size_t len = s.length();
	for(int i=0; i<len; ++i)
		for(int j=i+1; j<len; ++j)
			if(s[i] == s[j])
				return true;
	return false;
}

// check using counting every possible char
// technically O(n), efficient for large strings
// (much larger than CHAR_MAX)
bool duplicate_check_inplace_count(std::string s)
{
	size_t len = s.length();
		for(int i=0; i<=CHAR_MAX; ++i)
			if (std::count(s.begin(), s.end(), i) > 1)
				return true;
	return false;
}

// check by sorting the string and checking for consecutives
// this is kind of a cheap way of doing it.
bool duplicate_check_sort(std::string s) {
	std::sort(s.begin(), s.end());
	std::string::iterator it = std::adjacent_find(s.begin(), s.end());
	return (it == s.end()) ? false : true;
}


int main() 
{
	// ask user input
	std::cout << "Enter a string: ";
	std::string s;
	std::getline(std::cin, s);

	// check the length
	bool has_duplicate = duplicate_check_maphist(s);
	if (has_duplicate) 
		std::cout << "There is a duplicate!" << std::endl;
	else 
		std::cout << "There are no duplicates!" << std::endl;
}