Go: package main

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

var memo []int8

func solve(v int) int8 {
	if v == 6174 {
		return 0
	}
	if memo[v] == 0 {
		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([]int8, 10000)
	for {
		fmt.Scan(&v)
		if v == 0 {
			break
		}
		if v%1111 == 0 {
			fmt.Println("NA")
			continue
		}
		fmt.Println(solve(v))
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  string n;
  cin >>n;
  while(n!="0000"){
    int a[4];
    for(int i=0;i<4;i++){
      stringstream ss;
      ss <<n[i];
      ss >>a[i];
    }
    if((a[0]==a[1])&&(a[1]==a[2])&&(a[2]==a[3])){
      cout <<"NA"<<endl;
      cin >>n;
      continue;
    }
    for(int i=0;i<100;i++){
      if(n=="6174"){
        cout <<i <<endl;
        break;
      }
      sort(a,a+4);
      int l=a[0]+a[1]*10+a[2]*100+a[3]*1000;
      int s=a[3]+a[2]*10+a[1]*100+a[0]*1000;
      int n1=l-s;
      stringstream ss;
      ss << n1;
      ss >> n;
      if(n1<10){
        n.resize(4);
        n[3]=0;
        n[2]=0;
        n[1]=0;
      }else if(n1<100){
        n.resize(4);
        n[3]=0;
        n[2]=0;
      }else if(n1<1000){
        n.resize(4);
        n[3]=0;
      }
        for(int j=0;j<4;j++){
        stringstream ss1;
        ss1 <<n[j];
        ss1 >>a[j];
      }
    }
    cin >>n;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
main(){
  while(1){
    int n,l,s,cnt=0;
    bool check=true;
    int keta[4]={0};
    cin>>n;
    if(n==0){
      break;
    }
    keta[0]=(n)%10;
    keta[1]=(n/10)%10;
    keta[2]=(n/100)%10;
    keta[3]=(n/1000)%10;
    if(keta[0]==keta[1]&&keta[1]==keta[2]&&keta[2]==keta[3]){
      cout<<"NA"<<endl;
    }
    else{
      while(n!=6174){
	cout<<n<<endl;
	sort(keta,keta+4);
	s=keta[0]*1000+keta[1]*100+keta[2]*10+keta[3];
	l=keta[0]+keta[1]*10+keta[2]*100+keta[3]*1000;
	n=l-s;
	keta[0]=(n)%10;
	keta[1]=(n/10)%10;
	keta[2]=(n/100)%10;
	keta[3]=(n/1000)%10;
	cnt++;
      }
      cout<<cnt<<endl;
    }
  }
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <cfloat>
#include <ctime>
#include <cassert>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
#include <numeric>
#include <list>
#include <iomanip>
#include <fstream>
#include <bitset>
   
using namespace std;
   
 
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
#define foreach(it, c) for (__typeof__((c).begin()) it=(c).begin(); it != (c).end(); ++it)
#define rforeach(it, c) for (__typeof__((c).rbegin()) it=(c).rbegin(); it != (c).rend(); ++it)
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define CL(arr, val) memset(arr, val, sizeof(arr))
#define COPY(dest, src) memcpy(dest, src, sizeof(dest))
#define ten(n) ((long long)(1e##n))
#define bin(n) (1LL << (n))
#define erep(i, n) for (int i = 0; i <= (int)(n); ++i)
#define revrep(i, n) for (int i = (n); i >= 0; --i)
#define pb push_back

template <class T> void chmax(T& a, const T& b) { a = max(a, b); }
template <class T> void chmin(T& a, const T& b) { a = min(a, b); }

template <class T> T round_up(T a, T b) { return (a + b - 1) / b; }

template <class T> void uniq(vector<T>& c) { sort(c.begin(), c.end()); c.erase(unique(c.begin(), c.end()), c.end()); }
 
template <class T> string to_s(const T& a) { ostringstream os; os << a; return os.str(); }
template <class T> T to_T(const string& s) { istringstream is(s); T res; is >> res; return res; }
  
template <class T, class U> ostream& operator<<(ostream& os, pair<T, U>& p) { os << "( " << p.first << ", " << p.second << " )"; return os; }
  
template <class T> void print(T a, int n, const string& deli = " ", int br = 1) { for (int i = 0; i < n; ++i) { cout << a[i]; if (i + 1 != n) cout << deli; } while (br--) cout << endl; }
template <class T> void print(const T& c, const string& deli = " ", int br = 1) { foreach (it, c) { cout << *it; if (++it != c.end()) cout << deli;--it; } while (br--) cout << endl; }
template <class T> void print2d(T a, int w, int h, int width = -1, int br = 1) { for (int i = 0; i < h; ++i) { for (int j = 0; j < w; ++j) {    if (width != -1) cout.width(width); cout << a[i][j] << ' '; } cout << endl; } while (br--) cout << endl; }
   
template <class T> void input(T& a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }
template <class T> void input(T* a, int n) { for (int i = 0; i < n; ++i) cin >> a[i]; }

void fix_pre(int n) { cout.setf(ios::fixed, ios::floatfield); cout.precision(10); }
void fast_io() { cin.tie(0); ios::sync_with_stdio(false); }
#define trace(x) (cout << #x << ": " << (x) << endl)
 
bool in_rect(int x, int y, int w, int h) { return 0 <= x && x < w && 0 <= y && y < h; }
 

typedef long long ll;
typedef pair<int, int> pint;
 
// y(v): v>^<  y(^): ^>v<
const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { 1, 0, -1, 0 };

const double PI = acos(-1.0);

#define F first
#define S second


const int inf = ten(6);
int dp[11111];
int recur(int n)
{
    if (dp[n] != -1)
        return dp[n];

    string n_str = to_s(n);
    sort(all(n_str));
    int s = to_T<int>(n_str);
    reverse(all(n_str));
    int l = to_T<int>(n_str);

    return dp[n] = l - s == n  ? inf : recur(l - s) + 1;
}
int main()
{
    CL(dp, -1);
    dp[0] = inf;
    dp[6174] = 0;

    int n;
    while (cin >> n, n)
    {
        int r = recur(n);
        if (r < inf)
            cout << r << endl;
        else
            cout << "NA" << endl;
    }
}

C++: #include<bits/stdc++.h>
using namespace std;
main(){
  while(1){
    int n,l,s,cnt=0;
    bool check=true;
    int keta[4]={0};
    cin>>n;
    if(n==0){
      break;
    }
    keta[0]=(n)%10;
    keta[1]=(n/10)%10;
    keta[2]=(n/100)%10;
    keta[3]=(n/1000)%10;
    if(keta[0]==keta[1]&&keta[1]==keta[2]&&keta[2]==keta[3]){
      cout<<"NA"<<endl;
    }
    else{
      while(n!=6174){
	sort(keta,keta+4);
	s=keta[0]*1000+keta[1]*100+keta[2]*10+keta[3];
	l=keta[0]+keta[1]*10+keta[2]*100+keta[3]*1000;
	n=l-s;
	keta[0]=(n)%10;
	keta[1]=(n/10)%10;
	keta[2]=(n/100)%10;
	keta[3]=(n/1000)%10;
	cnt++;
      }
      cout<<cnt<<endl;
    }
  }
}

C++: #include<bits/stdc++.h>
using namespace std;
signed main(){
  cin.tie(NULL);
  ios::sync_with_stdio(false);
  long long n,keta[5],ns,nb,kaisu=0,a=4;
  while(true){
    cin>>n;
    if(n==0000) break;
    else{
      if(n%1111==0) cout<<"NA"<<'\n';
      else{
	while(true){
	  if(n==6174){
	    cout<<kaisu<<'\n';
	    kaisu=0;
	    break;
	  }
	  else{
	    kaisu++;
	    for(int i=0;i<4;i++){
	      keta[i]=n%10;
	      n=n/10;
	    }
	    sort(keta,keta +a);
	    ns=keta[0]*1000+keta[1]*100+keta[2]*10+keta[3];
	    nb=keta[3]*1000+keta[2]*100+keta[1]*10+keta[0];
	    n=nb-ns;
	  }
	}
      }
    }
  }
  return(0);
}