Go: package main

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

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 {
		var s string
		fmt.Scan(&s)
		v, _ = strconv.Atoi(s)
		if v == 0 {
			break
		}
		if v%1111 == 0 {
			fmt.Println("NA")
			continue
		}
		fmt.Println(solve(v))
	}
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define _USE_MATH_DEFINES
#include <iostream>
#include <cstdio>
#include <vector>
#include <string>
#include <array>
#include <queue>
#include <deque>
#include <set>
#include <list>
#include <map>
#include <stack>
#include <utility>
#include <bitset>
#include <complex>
#include <algorithm>
#include <numeric>
#include <cstdlib>
#include <cmath>
#include <cctype>
#include <cstring>
#include <climits>
#include <random>
#include <functional>
#include <sstream>
#include <iomanip>

using namespace std;

#define rep(i, n) for(int i=0; i<(n); i++)
#define FOR(i, m, n) for(int i=(m);i<(n);i++)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()
#define SORT(x) sort((x).begin(),(x).end())
#define REVE(x) reverse((x).begin(),(x).end())
#define mp make_pair
#define pb push_back
#define dump(x)  cout << #x << " = " << (x) << endl;
#define forauto(i, a) for(auto i:a)cout << i << " "; puts("");

typedef long long LL;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<string> VS;
typedef vector<bool> VB;
typedef vector<vector<int>> VVI;
typedef pair<int, int> PII;
typedef pair<double, double> PDD;
typedef complex<double> CD;

string aaa(string t) {
	string l = t; SORT(l); REVE(l);
	string s = t; SORT(s);
	int ans = stoi(l) - stoi(s);
	string sans = to_string(ans);
	string ret; ret.assign(4 - sz(sans), '0');
	ret += to_string(ans);
	return ret;
}

int main() {
	while (true) {
		string s; cin >> s;
		if (s == "0000")break;

		bool flag = (s[0] == s[1] && s[1] == s[2] && s[2] == s[3]);
		int ans = 0;
		while (s != "6174" && !flag) {
			s = aaa(s);
			ans++;
		}
		if (flag)puts("NA");
		else cout << ans << endl;
	}
}

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

int main(){
  int t[4];
  int tt[4];
  int a,b,bb,c;
  while(true){
    cin>>a;
    c=0;
    if(a==0)break;
    for(int i=1111;i<10000;i+=1111){
      if(i==a){
	cout<<"NA"<<endl;
	c=-1;
      }
    }
    if(c==-1)continue;
    while(a!=6174){
      b=a;
      c++;
      for(int i=3;i>=0;i--){
	t[i]=b%10;
	b/=10;
      }
      sort(t,t+4);
      reverse(t,t+4);
      b=t[0]*1000+t[1]*100+t[2]*10+t[3];

      bb=a;
      for(int i=3;i>=0;i--){
	t[i]=bb%10;
	bb/=10;
      }
      sort(t,t+4);
      bb=t[0]*1000+t[1]*100+t[2]*10+t[3];
      
      a=b-bb;
      cout<<a<<endl;
    } 
    cout<<c<<endl;
    
  }
  return 0;
}

C++: //ソートを使いたくなかっただけなんだ
#include <iostream>
#include <iomanip>
#include <string>
#include <sstream>
using namespace std;

int main()
{
	while (true)
	{
		string N;
		cin >> N;
		if (N == "0000")
		{
			break;
		}
		else if (N[0]==N[1] && N[1]==N[2] && N[2]==N[3])
		{
			cout << "NA" << endl;
			continue;
		}

		int cnt1 = 0;
		while (N != "6174")
		{
			int cnts[10];
			for (int i=0; i<10; i++)
			{
				cnts[i] = 0;
				unsigned int j = 0;
				while ((j=N.find('0'+i, j)) != string::npos)
				{
					++j;
					++cnts[i];
				}
			}
			string L="0000", S="0000";
			int cnt2 = 0;
			for (int i=0; i<10; i++)
			{
				for (int j=0; j<cnts[i]; j++)
				{
					L[3-cnt2] = S[cnt2] = '0'+i;
					++cnt2;
				}
			}

			stringstream ss_L(L), ss_S(S);
			int i_L, i_S;
			ss_L >> i_L;
			ss_S >> i_S;
			stringstream ss_N;
			ss_N << setfill('0') << setw(4) << i_L-i_S;
			N = ss_N.str();
			++cnt1;
		}
		cout << cnt1 << endl;
	}

	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <string>
#include <string.h>
#include <sstream>


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 <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b) - 1;i>=(a);i--)
#define REP(i,n) for(int i=0;i<(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)

#define PB push_back
#define INF INT_MAX/3
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CLR(a) memset(a,0,sizeof(a))

typedef long long int ll;

using namespace std;

int main(){
    while(true){
	string s;
	ll ans = 0;
	cin >> s;
	if(s == "0000") break;
	else if(s[0] == s[1] && s[0] == s[2] && s[0] == s[3]){
	    cout << "NA" << endl;
	    continue;
	}
	while(s != "6174"){
	    vector<int> a(4);
	    vector<int> b(4);
	    a[0] = (int)(s[0] - '0');
	    a[1] = (int)(s[1] - '0');
	    a[2] = (int)(s[2] - '0');
	    a[3] = (int)(s[3] - '0');
	    b[0] = (int)(s[0] - '0');
	    b[1] = (int)(s[1] - '0');
	    b[2] = (int)(s[2] - '0');
	    b[3] = (int)(s[3] - '0');
	    sort(ALL(b));
	    sort(RALL(a));
	    int an = 1000 * a[0] + 100 * a[1] + 10 * a[2] + a[3];
	    int bn = 1000 * b[0] + 100 * b[1] + 10 * b[2] + b[3];
	    s = to_string(an - bn);
	    ans++;
	}
	cout << ans << endl;
    }
    return 0;
}

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

int main(){
  int n;

  while(cin >> n, n){
    n++;
    int t[102];

    for(int i = 0; i < n; i++){
      cin >> t[i];
    }

    int val1 = -1, val2 = -1;
    int cnt1 = 0, cnt2 = 0;

    for(int i = 1; i < n; i++){
      int diff = t[i] - t[i - 1];

      if(val1 == -1){
        val1 = diff;
        cnt1++;
      }
      else if(val1 == diff){
        cnt1++;
      }
      else{
        val2 = diff;
        cnt2++;
      }
    }

    if(cnt2 > cnt1){
      swap(val1, val2);
    }

    for(int i = 1; i < n; i++){
      int diff = t[i] - t[i - 1];

      if(val1 != diff){
        if(i == 1 && t[2] - t[1] == val1){
          cout << t[0] << endl;
        }
        else{
          cout << t[i] << endl;
        }
        break;
      }
    }
  }
}