Go: package main

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

var memo []int

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

C++: #include <cstdio>
#include <algorithm>

using namespace std;

int main()
{
	int N[4], L[4], S[4], l, s, i, num;
	char input[5];
	
	while (1){
		scanf("%s", input);
		
		for (i = 0; i < 4; i++){
			N[i] = input[i] - '0';
		}
		
		if (input[0] == '0' && input[1] == '0' && input[2] == '0' && input[3] == '0'){
			break;
		}
		
		if (input[0] == input[1] && input[0] == input[2] && input[0] == input[3]){
			printf("NA\n");
			continue;
		}
		
		num = 0;
		while (1){
			if (N[0] == 6 && N[1] == 1 && N[2] == 7 && N[3] == 4){
				break;
			}
			
			for (i = 0; i < 4; i++){
				L[i] = N[i];
				S[i] = N[i];
			}
			
			sort(L, L + 4);
			reverse(L, L + 4);
			sort(S, S + 4);

			for (i = 3; i >= 0; i--){
				N[i] = L[i] - S[i];
				if (N[i] < 0){
					N[i] += 10;
					L[i - 1]--;
				}
			}
			
			num++;
		}
		
		printf("%d\n", num);
	}
	
	return (0);
}

C++: #include <cstdio>
#include <cstring>
#include <string>
#include <iostream>

using namespace std;

int main(){

	while(1){
		int n=0,l,s;
		int cnt = 0;
		string tmp;

		cin >> tmp;
		n = atoi(tmp.c_str());

		if(n == 0)break;
		if(n % 1111 == 0){
			printf("NA\n");
			continue;
		}

		while(n != 6174){
			sort(tmp.begin(),tmp.end());
			s = atoi(tmp.c_str());

			sort(tmp.begin(),tmp.end(),greater<char>());
			l = atoi(tmp.c_str());
			n = l - s;

			for(int i=0,p=1000;i<4;i++,p/=10){
				tmp[i] = '0' + n/p;
				n %= p;
			}
			n = atoi(tmp.c_str());
			cnt++;
		}
		printf("%d\n", cnt);
	}
	return 0;
}

C++: #include <bits/stdc++.h>
typedef long long lint;
using namespace std;
#define rep(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)

int f(string n) {
    int cnt = 0;
    while (n != "6174") {
        sort(n.begin(),n.end());
        int l = stoi(n);
        sort(n.begin(),n.end(), greater<int>());
        int s = stoi(n);
        if (s == l) {
            return -1;
        }
        ostringstream str;
        str << setfill('0') << setw(4) << s-l;
        n = str.str();
        cnt ++;
    }
    return cnt;
}

bool solve() {
    string n; cin >> n;
    if (n == "0000") return false;
    int cnt = f(n);
    if (cnt == -1) cout << "NA" << endl;
    else cout << cnt << endl;

    return true;
}

int main() {
    while (solve()) {}
}

C++: // 2014/09/05 Tazoe

#include <iostream>
#include <algorithm>
using namespace std;

int main()
{
	while(true){
		int N;
		cin >> N;

		if(N==0)
			break;

		if(N%1111==0){		// 全桁が同じ数字
			cout << "NA" << endl;
			continue;
		}

		int cnt = 0;

		while(N!=6174){
			int n[4];
			n[0] = N/1000;
			n[1] = N/100%10;
			n[2] = N/10%10;
			n[3] = N%10;

			sort(n, n+4);

			int L = n[3]*1000+n[2]*100+n[1]*10+n[0];
			int S = n[0]*1000+n[1]*100+n[2]*10+n[3];

			N = L-S;
			cnt++;
		}

		cout << cnt << endl;
	}

	return 0;
}

C++: #include <vector>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>
#include <utility>
#include <functional>
#include <sstream>
#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <climits>
#include <fstream>
using namespace std;
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toStr(T x) { ostringstream sout; sout << x; return sout.str(); }
typedef vector<int> vi;
typedef vector<vi>  vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define FOR(i,a,b) for(int i=(a);i<=(b);++i)
#define REP(i,n) FOR(i,0,(n)-1)
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = INT_MAX / 2;
const int MOD = 1000000007;

using namespace std;
typedef vector<ll> vl;
typedef vector<vl> vvl;

int main() {
	string s;
	while (cin >> s, s != "0000") {
		int ans = 0;
		if (toInt(s) % 1111 == 0) {
			cout << "NA" << endl;
		}
		else {
			while (s != "6174") {
				string a = s, b = s;
				sort(ALL(a));
				sort(RALL(b));
				s = toStr(toInt(b) - toInt(a));
				ans++;
			}
			cout << ans << endl;
		}
	}
	return 0;
}

C++: #include<iostream>
#include<sstream>
#include<algorithm>
using namespace std;
inline 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()
{
  string n;
  for(;cin>>n,n!="0000";)
    {
      int cnt=0;
      while(n!="6174")
	{
	  string s=n;
	  string l=n;
	  sort(s.begin(),s.end());
	  sort(l.begin(),l.end(),greater<char>());
	  n=toString(toInt(l)-toInt(s));
	  while(n.size()<4)
	    n="0"+n;
	  if(s==l)
	    {
	      cnt=-1;
	      break;
	    }  
	  cnt++;  
	}
      if(cnt>=0)
	cout<<cnt<<endl;
      else 
	cout<<"NA"<<endl;
    }
}

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.compare("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(RALL(a));
	    sort(ALL(b));
	    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];
	    if(an < bn) swap(an,bn);
	    s = to_string(an - bn);
	    ans++;
	}
	cout << ans << endl;
    }
    return 0;
}

C++: #include <iostream>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-8
#define F first
#define S second

static const double PI=6*asin(0.5);
typedef long long ll;
typedef complex<double> CP;
typedef pair<long long,int> P;
static const int INF=1<<24;

string s;

int main(){
	while(cin>>s,s!="0000"){
		if(s=="6174"){
			cout<<0<<endl;
			continue;
		}
		if(s[0]==s[1]&&s[1]==s[2]&&s[2]==s[3]){
			cout<<"NA\n";
			continue;
		}
		int c=0;
		while(1){
		int MIN,MAX;
		vector <int> v;
		rep(i,4){
			s[i]-='0';
			v.PB(s[i]);
		}
		MIN=0;MAX=0;
		sort(v.begin(),v.end());
		rep(i,4){
			MIN*=10;
			MIN+=v[i];
		}
		sort(v.begin(),v.end(),greater<int>());
		rep(i,4){
			MAX*=10;
			MAX+=v[i];
		}
			int a=MAX-MIN;
			if(MAX-MIN==6174) break;
			
			else{
				rep(i,4){
					s[3-i]=a%10;
					a/=10;
				}
				c++;
			}
		}
		cout<<c+1<<endl;
	}
	return 0;
}