Display duplicate characters in a string - c++

Question

I wrote some code in C++ to display duplicate characters in a string, but if a character is repeated more than three times, the code prints the repeated character more than once.
For example if the string is aaaddbss, it should only print out ads but it prints aaads instead.
What am I doing wrong?
cout << " Please enter a string" << endl;
cin.getline(input, 100); // example input (ahmad wahidy) the output reads a a h a d instead of a h d
for (int i = 0;input[i]!='\0'; i++)
{
for (int j = i+1;input[j]!='\0'; j++)
{
if (input[i] == input[j])
{
cout << input[i] << " ";
}
}
}
cout << endl;

Answer 1

Instead of using your own custom methods, why not use a short and standard method?
Given an std::string input with the text, this will print the unique chars:
std::set<char> unique(input.begin(), input.end());
for (auto & c : unique)
{
std::cout << c << " ";
}
std::cout << std::endl;

Answer 2

You can use std::count and std::set:
#include <string>
#include <set>
#include <iostream>
using namespace std;
int main()
{
string s = "hellohowareyou";
set<char>the_set(s.begin(), s.end());
for (char i:the_set)
if (count(s.begin(), s.end(), i) > 1)
cout << i << endl;
}
Output:
e
h
l
o

Answer 3

If you are not allowed to use a map (and probably also not allowed to use a set), you could simply use an array of integers to count occurrences, with one entry for each possible char value. Note that a character - when taken as an ASCII value - can be directly used as an index for an array; however, to avoid negative indices, each character value should first be converted to an unsigned value.
#include <iostream>
#include <limits>
int main() {
const char* input = "aaaddbss";
int occurrences[UCHAR_MAX+1] = { 0 };
for (int i = 0;input[i] !='\0'; i++)
{
unsigned char c = input[i];
if (occurrences[c]==0) {
occurrences[c]++;
}
else if (occurrences[c]==1) {
occurrences[c]++;
cout << "duplicate: " << c << endl;
}
}cout << endl;
}