First non-repeating character in a stream #61.txt

#include <bits/stdc++.h>

using namespace std;

string FirstNonRepeating(string str)      //Function definition of FirstNonRepeating function
{
    vector<char> a;     //Declare a vector a
    string ans = "";    //Declare a string ans for storing the answer
    unordered_map<char, int> mpp;    //Declare an unordered map to keep count of frequency of characters in string

    for(int i=0; i<str.length(); i++)   //Trace the string
    {
        if(mpp[str[i]] > 0)      //Check if the frequency of that character in > 0
        {
            if(a.empty() == 0 && find(a.begin(), a.end(), str[i]) != a.end())  //Find the character in vector a
            {
                a.erase(find(a.begin(), a.end(), str[i]));     //Erase the character in vector a
            }

            if(a.empty())     //Check if vector a is empty
            {
                ans += '#';   //Append # to string ans
            }

            else
            {
                ans += a[0];   //Append a[0] to ans string
            }
        }

        else   //If frequency is 0
        {
            mpp[str[i]]++;  //Increase the frequency
            a.push_back(str[i]);   //Push that character in vector a
            ans += a[0];     //Append a[0] to ans string
        }
    }

    return ans;    //Return ans string
}

int main()
{
    string str;

    cout<<"Enter a String in Lower Case : "<<endl;    //Take string in lower case characters
    cin>>str;

    for(int i=0; i<str.length(); i++)
    {
        if(isupper(str[i]))  //Check if that string has any upper case characters
        {
            cout<<"\nInvalid String!"<<endl;
            return 0;
        }
    }

    cout<<"\nOutput : "<<FirstNonRepeating(str)<<endl;      //Call the FirstNonRepeating function

    return 0;
}