Go: package main

import (
	"fmt"
	"sort"
	"strconv"
)

var memo map[int]int

func solve(v int) int {
	if v == 6174 {
		return 0
	}
	if _, ok := memo[v]; !ok {
		str := ([]byte)(strconv.Itoa(v))
		sort.Slice(str, func(i, j int) bool {
			return str[i] < str[j]
		})
		s, _ := strconv.Atoi(string(str))
		sort.Slice(str, func(i, j int) bool {
			return str[i] > str[j]
		})
		l, _ := strconv.Atoi(string(str))
		memo[v] = solve(l-s) + 1
	}
	return memo[v]
}

func main() {
	var v int
	memo = make(map[int]int)
	for {
		fmt.Scan(&v)
		if v == 0 {
			break
		}
		if v%1111 == 0 {
			fmt.Println("NA")
			continue
		}
		fmt.Println(solve(v))
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
using namespace std;
int main() {
	char S[5];
	while(scanf("%s",S)) {
		if(strcmp(S,"0000")==0) break;
		int ans=0;
		bool NG=0;
		while(strcmp(S,"6174")) {
			sort(S,S+4);
			NG=1;
			for(int i=0;i<3;i++) {
				if(S[i]!=S[i+1]) NG=0;
			}
			if(NG) break;
			int a=0,b=0,c=1;
			for(int i=0;i<4;i++) {
				a+=(S[i]-'0')*c;
				b+=(S[3-i]-'0')*c;
				c*=10;
			}
			int k=a-b;
			for(int i=3;i>=0;i--) {
				S[i]=(k%10+'0');
				k/=10;
			}
			ans++;
		}
		if(NG) printf("NA\n"); else printf("%d\n",ans);
	}
}

C++: #include <cassert>

#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
//#include <boost/foreach.hpp>
//#include <boost/range/algorithm.hpp>
#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define ll long long
#define Sort(v) sort(all(v))
#define INF 1e9
#define LINF 1e18
#define END return 0
#define pb push_back
#define se second
#define fi first
#define pb push_back
#define all(v) (v).begin() , (v).end()
#define MP make_pair
#define MOD 1000000007LL
#define int long long
using namespace std;
int day[12]={31,28,31,30,31,30,31,31,30,31,30,31};
int dx[]={0,1,0,-1};
int dy[]={1,0,-1,0};
struct edge{int to,cost;};
//typedef pair<int,int> P;

bool isupper(char c){if('A'<=c&&c<='Z')return 1;return 0;}
bool islower(char c){if('a'<=c&&c<='z')return 1;return 0;}
bool isPrime(int x){if(x==1)return 0;if(x==2)return 1;if(x%2==0)return 0;for(int i=3;i*i<=x;i++)if(x%i==0)return 0;return 1;}
bool iskaibun(string s){for(int i=0;i<s.size()/2;i++)if(s[i]!=s[s.size()-i-1])return 0;return 1;}
bool isnumber(char c){return ('0'<=c&&c<='9');}
bool isalpha(char c){return (isupper(c)&&islower(c));}
void printvi(vector<int> v){rep(i,0,v.size()){if(i)cout<<" ";cout<<v[i];}cout<<endl;}
void printvil(vector<int> v){rep(i,0,v.size()){cout<<v[i]<<endl;}}
void printvvi(vector<vector<int>> v){
    rep(i,0,v.size()){
        rep(j,0,v[i].size()){
            if(j)cout<<" ";
            cout<<v[i][j];
        }
        cout<<endl;
    }
}
void printvstr(vector<string> v){
    rep(i,0,v.size()){
        cout<<v[i]<<endl;
    }
}

signed main (){
    string s;
    while(1){
        cin>>s;
        if(s=="0000")break;
        auto cmp=[&](char a,char b){
            return a>b;
        };
        if(s[0]==s[1] and s[1]==s[2] and s[2]==s[3]){
            cout<<"NA"<<endl;
            continue;
        }
        int i=0;
        for(;s!="6174";i++){
            sort(all(s));
            int a1=stol(s);
            sort(all(s),cmp);
            int a2=stol(s);
            int a3=a2-a1;
            s=to_string(a3);
        }
        cout<<i<<endl;

    }

    
}
/*
*/

C++: #include<iostream>
#include<algorithm>
#include<functional>
#include<string>
#include<sstream>
using namespace std;

template<class T, class U>
T lexical_cast(U &obj){
    T ret;
    stringstream ss;
    ss << obj;
    ss >> ret;
    return ret;
}

int main(){
  while(1){
    string N, L, S;
    cin >> N;
    if(N == "0000") break;
    
    int ans = 0;
    while(1){
      if(N == "6174"){
        cout << ans << endl;
        break;
      }
      
      L = N;
      S = N;
      
      sort(L.begin(), L.end(), greater<char>());
      sort(S.begin(), S.end());
      
      if(L == S){
        cout << "NA" << endl;
        break;
      }
      
      int n = lexical_cast<int>(L) - lexical_cast<int>(S);
      N = lexical_cast<string>(n);
      
      ans++;
    }
  }
  return 0;
}

C++: #include<iostream>
#include<algorithm>
using namespace std;


bool check(char data[4]){
  for(int i = 1 ; i < 4 ; i++){
    if(data[0] != data[i])return false;
  }
  return true;
}

int toInt(char data[4]){
  int ret = 0;
  for(int i = 0 ; i < 4 ; i++){
    ret += (data[i]-'0');
    ret *= 10;
  }  
  return ret/10;
}

void getNext(int a,char data[4]){
  for(int i = 0 ; i < 4 ; i++){
    data[3 - i] = a%10 + '0';
    a /= 10;
  }
}

void solve(char data[4]){

  if(check(data)){
    cout << "NA" << endl;
    return;
  }

  if(toInt(data) == 6174){
    cout << 0 << endl;
    return;
  }

  int res;
  for(res = 1 ; ; res++){
    int L,S;
    sort(data,data + 4);
    S = toInt(data);
    reverse(data,data + 4);
    L = toInt(data);
    if(L - S == 6174)break;
    getNext(L - S,data);
  }
  cout << res << endl;
}

int main(){

  char data[4];
  while(1){
    bool f = true;
    for(int i = 0 ; i < 4 ; i++){
      cin >> data[i];
      if(data[i] != '0')f = false;
    }
    if(f)break;
    solve(data);
  }
  return 0;
}