include

include

using namespace std;

int main() { int t; cin >> t; while (t--) { int n; cin >> n; string s; cin >> s;

    stack<char> st;
    bool valid = true;

    for (int i = 0; i < n; i++) {
        if (s[i] == '.') {
            continue;
        }
        else if (s[i] == 'H') {
            if (!st.empty() && st.top() == 'H') {
                valid = false;
                break;
            }
            st.push('H');
        }
        else {
            if (!st.empty() && st.top() == 'T') {
                valid = false;
                break;
            }
            if (st.empty()) {
                valid = false;
                break;
            }
            st.pop();
        }
    }

    if (!st.empty()) {
        valid = false;
    }

    if (valid) {
        cout << "Valid" << endl;
    }
    else {
        cout << "Invalid" << endl;
    }
}
return 0;

}

Explanation:

1. The code begins by including the necessary libraries, iostream and stack, which are required for input/output and stack operations respectively.

2. The main function is defined, which is the entry point of the program.

3. The variable 't' is declared to store the number of test cases and is taken as input from the user.

4. A while loop is used to iterate through each test case until 't' becomes zero.

5. Inside the loop, the variable 'n' is declared to store the length of the snake procession and is taken as input from the user.

6. Another variable 's' of type string is declared to store the input string representing the snake procession.

7. A stack 'st' is declared to store the elements of the snake procession.

8. A boolean variable 'valid' is initialized to true, which will be used to determine if the snake procession is valid or not.

9. A for loop is used to iterate through each character of the input string.

10. If the current character is '.', it means there is no snake in that position, so the loop continues to the next iteration.

11. If the current character is 'H', it means a head of the snake is encountered. The code checks if the stack is not empty and if the top element of the stack is also 'H'. If so, it means two consecutive snake heads are encountered, which is invalid. In such a case, the 'valid' variable is set to false, and the loop is terminated using the 'break' statement.

12. If the current character is 'T', it means a tail of the snake is encountered. The code checks if the stack is not empty and if the top element of the stack is 'T'. If so, it means a tail is encountered without a corresponding head, which is invalid. In such a case, the 'valid' variable is set to false, and the loop is terminated using the 'break' statement.

13. If the current character is 'T' and the stack is empty, it means a tail is encountered without any previous head, which is invalid. In such a case, the 'valid' variable is set to false, and the loop is terminated using the 'break' statement.

14. If none of the above conditions are true, it means the current character is 'T' and a corresponding head is found in the stack. In this case, the tail is removed from the stack using the 'pop' operation.

15. After the loop ends, if the stack is not empty, it means there are remaining heads in the stack without corresponding tails, which is invalid. In such a case, the 'valid' variable is set to false.

16. Finally, the 'valid' variable is checked, and if it is true, it means the snake procession is valid. Otherwise, it is invalid. The respective message is printed to the console.

17. The main function ends, and the program returns 0 to indicate successful execution.