How to find the duplicate - c++

Question

/* I have written a code for a simple hangman based on instructions given, I just want to know how to detect the duplicated inputs. For this code, It only detects correct letters and the length
For example:
user input: HAM
H
A
A
HANGMAN! */
#include <iostream>
#include <cstring>
using namespace std;
int main()
{
// Word to be guessed can be assumed to be at most 20 characters long (including end-of-string null character)
char HangmanWord[20];
// Variable to store current guess
char GuessLetter;
int count = 0;
std::cin >> HangmanWord;
//char guessedletter[strlen(HangmanWord)];
//int j = 0;
bool valid = true;
// Check that input word only consists of uppercase English letters
for (int i = 0; i < strlen(HangmanWord); i++)
{
if ((HangmanWord[i] > 'Z') || (HangmanWord[i] < 'A'))
{
valid = false;
break;
}
}
while (valid == true)
{
std::cin >> GuessLetter; //Takes the Guess letter
if ((GuessLetter > 'Z') || (GuessLetter < 'A')) //Check that guessed letter is uppercase
{
break;
}
else
{
for (int i = 0; i < strlen(HangmanWord); i++) //loop to check every letter in word
{
if (GuessLetter == HangmanWord[i]) { //if the letter is equal to any letter in the word the count increases
count++;
break;
}
if (i == strlen(HangmanWord) - 1) { //If no letter is equal to any letter in the word the program exits
valid = false;
}
}
if (count == strlen(HangmanWord)) { //Checks if all letters were guessed and exits the loop after
valid = false;
}
}
}
if (count == strlen(HangmanWord)) { //Checks if all letters were guessed correct to print Hangman
cout << "HANGMAN!\n";
valid = false;
}

Answer 1

If you want to detect whether a letter has already been input, you can work with ASCII characters being represented by numbers. Assuming we limit ourselves to uppercase letters, 'A' is 65 and 'Z' is 90.
We can do math with these. 'A' - 'A' is 0 and 'C' - 'A' is 2.
We also know that arrays are indexed starting from zero.
We can put these things together to create an array of boolean values, and flag them as letters are guessed.
std::array<std::bool, 26> already_guessed;
std::fill(already_guess.begin(), already_guessed.end(), false);
And we can then check on whether a letter has been guessed by looking it up. Consider, for instance, checking whether 'D' has been guessed. They should start out false, as nothing has been guessed at the beginning.
already_guessed['D' - 'A']
If it hasn't, we can mark it as guessed:
already_guessed['D' - 'A'] = true