Go: package main

import (
	"fmt"
	"sort"
)

var memo []int

func solve(v int) int {
	if v == 6174 {
		return 0
	}
	if memo[v] == 0 {
		a := make([]int, 4)
		for i := 0; i < 4; i++ {
			a[i] = v % 10
			v /= 10
		}
		sort.Slice(a, func(i, j int) bool {
			return a[i] > a[j]
		})
		l := int(0)
		for i := 0; i < 4; i++ {
			l *= 10
			l += a[i]
		}
		sort.Slice(a, func(i, j int) bool {
			return a[i] < a[j]
		})
		s := int(0)
		for i := 0; i < 4; i++ {
			s *= 10
			s += a[i]
		}
		memo[v] = solve(l-s) + 1
	}
	return memo[v]
}

func main() {
	var v int
	memo = make([]int, 10000)
	for {
		fmt.Scan(&v)
		if v == 0 {
			break
		}
		if v%1111 == 0 {
			fmt.Println("NA")
			continue
		}
		fmt.Println(solve(v))
	}
}

C++: #define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;

//#define int long long
#define DBG 1
#define dump(o) if(DBG){cerr<<#o<<" "<<o<<endl;}
#define dumpc(o) if(DBG){cerr<<#o; for(auto &e:(o))cerr<<" "<<e;cerr<<endl;}
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define each(it,c) for(auto it=(c).begin();it!=(c).end();it++)
#define all(c) c.begin(),c.end()
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9 + 7);
bool NA(string s) {
	if (s == "1111" || s == "2222" || s == "3333" || s == "4444" || s == "5555" || s == "6666" || s == "7777" || s == "8888" || s == "9999")return true;
	else return false;
}
signed main() {
	string s;
	while (cin >> s, s != "0000") {
		int ans(0);
		while (s != "6174") {
			if (NA(s)) {
				cout << "NA" << endl;
				goto here;
			}
			string big(s), small(s);
			sort(all(small));
			sort(all(big), greater<char>());
			s = to_string(stoi(big) - stoi(small));
			ans++;
		}
		cout << ans << endl;
	here:;

	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <math.h>
#include <cmath>
#include <stdio.h>
#include <string>
#include <string.h>
#include <cstring>
#include <sstream>
#include <iomanip>
#include <stdlib.h>

using namespace std;

int main (void)
{
	int y = 0;
	for (long u = 0; u < 10000; u++)
	{
		int co = 0;
		string a;
		int g[4];

		cin >> a;
		for (int k = 0; k < 100; k++)
		{
			if (a == "6174")
			{
				cout << co << endl;

				break;
			}
			if (a == "0000")
			{
				y = -1;
				break;
			}

			for (int i = 0; i < 4; i++)
			{
				g[i] = 0;
				g[i] = stoi (a.substr(i,1));
			}
			if (g[0] == g[1] && g[1] == g[2])
			{
				if (g[2] == g[3])
				{
					cout << "NA" << endl;
					break;
				}
			}
			long Max = 0,Min = 0;

			sort(g,g + 4);

			Min = (g[0] * 1000) + (g[1] * 100) + (g[2] * 10) + g[3];
			Max = (g[3] * 1000) + (g[2] * 100) + (g[1] * 10) + g[0];

			co += 1;
			if (Max - Min == 6174)
			{
				cout << co << endl;
				break;
			}
			ostringstream p;
			p << Max - Min;
			a = p.str();
		}
		if (y != 0)
		{
			break;
		}
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#include <sstream>
#include <cstdio>
#include <cassert>

using namespace std;

int toInt(string s){int v;istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

int main(void)
{
	string n;
	while (cin >> n && n != "0000"){
		if (count(n.begin(), n.end(), n[0]) == 4){
			cout << "NA" << endl;
		}
		// assert(n != "0000");

		int tmp = toInt(n);
		char str[10];
		sprintf(str, "%04d", tmp);
		n = string(str);
		int cnt = 0;
		while (n != "6174"){
			if (count(n.begin(), n.end(), n[0]) == 4){
				cout << "NA" << endl;
				goto END;
			}
			string L = n; sort(L.rbegin(), L.rend());
			string S = n; sort(S.begin(), S.end());
			// cout << n << " " << L << " " << S << endl;
			int tmp = toInt(L) - toInt(S);
			char str[10];
			sprintf(str, "%04d", tmp);
			n = string(str);
			cnt++;
		}
		cout << cnt << endl;
		END:;
	}

	return 0;
}

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

int main() {


  string n;

  while(1) {
    cin >>n;
    
    int flg = 0;
    for(int i=0;i<4;i++) if(n[i] != '0') flg =1;
    if(flg == 0) break;
    
    int a[4];
    for(int i=0;i<4;i++) a[i] = n[i]-'0';
    int N=0;
    int t =1;
    for(int i=0;i<4;i++) N += a[3-i]*t,t*=10;
    int ans = 0;
    while(1) {
      if( N == 6174 || ans > 10000) break;
      ans++;
      sort(a,a+4);
      int maxn = 0;
      int minn = 0;
      t = 1;
      for(int i=0;i<4;i++) {
	maxn += a[i] * t;
	minn += a[3-i] * t;
	t*=10;
      }
      
      N = maxn - minn;
      t = N;
      for(int i=0;i<4;i++) a[i] = t%10, t/=10;
    }
    if(ans > 10000) cout <<"NA"<<endl; 
    else     cout << ans << endl;
  }

    
    return 0;    
    
    
}

C++: #include <bits/stdc++.h>
using namespace std;

int main () {
    string str;
    while (cin >> str) {
        bool f = false;
        for (auto i : str) if (i != '0') f = true;

        if (!f) break;

        f = false;
        for (auto i : str) if (i != str[0]) f = true;

        if (!f) {
            cout << "NA" << endl;
            continue;
        }

        int count = 0;
        int now;
        stringstream org;
        org << str;
        org >> now;
        while (1) {
            if (now == 6174) {
                cout << count << endl;
                break;
            }
            count++;
            sort(str.begin(), str.end());
            auto rstr = str;
            reverse(rstr.begin(), rstr.end());

            stringstream ss;
            ss << rstr << " " << str;
            int A, B;
            ss >> A >> B;
            now = A - B;
            stringstream next;
            next << now;
            next >> str;
        }
    }

    return 0;
}

C++: //============================================================================
// Name        : AOJ.cpp
// Author      :
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
#include <cstdio>
#include <string>
#include <cctype>
using namespace std;

int main() {
	string n,L,S;
	int R=0;
	while(1){
		cin>>n;
		if(n=="0000")break;
		if(n=="6174")cout<<"0"<<endl;
		L=S=n;
		sort(L.begin(),L.end(),greater<char>());
		sort(S.begin(),S.end());

		cout<<R<<endl;
	}



	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  int m,n,l;
  cin>>n;
  while(n!=0){
    int nn[4]={n%10,(n/10)%10,(n/100)%10,(n/1000)%10};
    //cout<<nn[3]<<nn[2]<<nn[1]<<nn[0]<<endl;
    if(nn[0]==nn[1]&&nn[1]==nn[2]&&nn[2]==nn[3]){
      cout<<"NA"<<endl;
    }else{
      int cou=0;
      while(nn[0]!=4||nn[1]!=7||nn[2]!=1||nn[3]!=6){
	int rn[4];
	for(int i=0;i<3;i++){
	  for(int j=i+1;j<4;j++){
	    if(nn[i]>nn[j]){
	      int exi=nn[i];
	      nn[i]=nn[j];
	      nn[j]=exi;
	    }
	  }
	}
	//cout<<nn[3]<<nn[2]<<nn[1]<<nn[0]<<' '<<nn[0]<<nn[1]<<nn[2]<<nn[3]<<endl;
	for(int i=0;i<4;i++){
	  rn[i]=nn[i];
	}
	for(int i=0;i<4;i++){
	  if(rn[i]-nn[3-i]<0){
	    rn[i]+=10;
	    rn[i]-=nn[3-i];
	    rn[i+1]--;
	  }else{
	    rn[i]-=nn[3-i];
	  }
	}
	for(int i=0;i<4;i++){
	  nn[i]=rn[i];
	}
	//cout<<nn[3]<<nn[2]<<nn[1]<<nn[0]<<endl;
	cou++;
      }
      cout<<cou<<endl;
    }
    cin>>n;
  }
  return (0);
}