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//presented by Maskros - 2021/05/20
#include<bits/stdc++.h>
using namespace std;
vector<char> s;	//Save the original text
string s2=""; //Save the original text after encoding
map<char,int> mp;	//Count the number of occurrences of letters
map<char,string> hfcode; //Save Huffman Code
int len,len2;	//Text length
map<char,int>::iterator it;
map<char,string>::iterator it2;
struct Node{
	int value;
	char alpha;
	Node *lchild,*rchild;
	string code;
	Node(){}
	Node(int v,char a,Node *l,Node *r):value(v),alpha(a),lchild(l),rchild(r){code="";}
};
Node *HTroot;
void write_file(char *name){	//Write file
	cout<<"Print '*' to stop typing...."<<endl<<endl;
	char ch;
	if(FILE *fp=fopen(name,"w")){
		while(1){
			ch=getchar();
			if(ch=='*') break; //while read '*' then end input
			fputc(ch,fp);
			s.push_back(ch);
		}
		fclose(fp);
	}
}
void cal_alpha(vector<char> v,int l){	//Count the number of words appearing in a letter
	for(int i=0;i<l;i++){
		if((v[i]<='Z'&&v[i]>='A')||(v[i]<='z'&&v[i]>='a'))
			mp[v[i]]++;
	}
}
void print_times(){	//Print the number of occurrences and the number of each letter
	it=mp.begin();
	putchar('n');
	cout<<"n="<<mp.size()<<endl;
	while(it!=mp.end()){
		cout<<it->first<<": "<<it->second<<endl;	
		it++;
	}
	putchar('n');
}
struct cmp{	//Used for priority queue sorting
	bool operator()(Node *a,Node *b){
		return a->value>b->value;
	}
};
void create_HT(){	//Build Huffman Tree
	priority_queue<Node*,vector<Node*>,cmp> q;
	for(it=mp.begin();it!=mp.end();it++){
		q.push(new Node(it->second,it->first,NULL,NULL));
	}
	if(mp.size()==1){HTroot=q.top(); return;}	//Huffman tree has only one root node
	Node *left,*right;
	while(1){
		left=q.top(); q.pop();
		right=q.top(); q.pop();
		if(q.empty()){
			HTroot=new Node(left->value+right->value,'0',left,right);
			break;
		}	
		q.push(new Node(left->value+right->value,'0',left,right));
	}
}
void encode_HT(Node *p,string ss){ //Coding the Huffman tree
	if(!p) return;
	p->code=ss;
	if(p->alpha!='0'){
		if(p->code.length()==0){ hfcode[p->alpha]="1"; return;}	//If the Huffman tree has only one root node, the default is 1
		hfcode[p->alpha]=p->code;		
	}
	encode_HT(p->lchild,ss+"0");
	encode_HT(p->rchild,ss+"1");
}
void print_hfcode(){	//Print Huffman code
	for(it2=hfcode.begin();it2!=hfcode.end();it2++){
		cout<<it2->first<<" => "<<it2->second<<endl;
	}
}
void encode_Text(char *name){ //Write the encoded original text to the file
	char tmp[100];
	string st;
	if(FILE *fp=fopen(name,"w")){
		for(int i=0;i<len;i++){	
			st=hfcode[s[i]];
			s2+=st;
			strcpy(tmp,st.c_str());	//String to char array
			fputs(tmp,fp);
		}
		fclose(fp);
	}
}
void decode_Text(char *name){	//Decode b.txt and save in c.txt
	Node *tmp=HTroot;
	char decode[10000];
	int j=0;
	if(mp.size()==1){	//If the Huffman tree has only one root node
		for(int i=0;i<s2.length();i++){
			if(tmp->alpha!='0') decode[j++]=tmp->alpha;		
		}
	}
	else{
		for(int i=0;i<=s2.length();i++){
			if(tmp->alpha!='0'){
				decode[j++]=tmp->alpha;
				tmp=HTroot;
			}
			if(s2[i]=='0') tmp=tmp->lchild;
			else tmp=tmp->rchild;
		}
	}
	if(FILE *fp=fopen(name,"w")){	
		for(int i=0;i<j;i++) fputc(decode[i],fp);
		fclose(fp);
	}
}
int main(){
	char a[]="a.txt";
	char b[]="b.txt";
	char c[]="c.txt";
	write_file(a);
	len=s.size();
	cal_alpha(s,len);
	print_times();
	create_HT();
	encode_HT(HTroot,"");
	print_hfcode();
	encode_Text(b);
	decode_Text(c);
	return 0;
}

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Test 1
The quick brown fox
jumps
over the lazy dog.

Test 2
aaaaa

Test 3
bbcbaaaad