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++: #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};
    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;
	    }
	  }
	}
	for(int i=0;i<4;i++){
	  rn[i]=nn[i];
	}
	for(int i=0;i<4;i++){
	  if(nn[3-i]>nn[i]){
	    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];
	}
	cou++;
      }
      cout<<cou<<endl;
    }
    cin>>n;
  }
  return (0);
}

C++: #include<stdio.h>
int main(void)
{
	int n,r[4],l,s,x,y,z,a,i,j,flg;
	while(1){
		scanf("%d",&n);
		if(n==0000){
			break;
		}
		flg=1;
		a=0;
		while(n!=6174){
			y=0;
			x=1000;
			for(i=0;i<4;i++){
				r[i]=n/x;
				n=n%x;
				x=x/10;
				if(r[0]==r[i]) y++;
			}
			if(y==4) {
				flg=0;
				break;
			}
			l=0; s=0;
			for(i=0;i<3;i++){
				for(j=i+1;j<4;j++){
					if(r[i]>r[j]){
						z=r[i];
						r[i]=r[j];
						r[j]=z;
					}
				}
			}
			printf("r==");
			for(i=0;i<4;i++) printf("%d",r[i]);
			printf("\n");
			x=1000;
			for(i=0;i<4;i++){
				s=r[i]*x+s;
				x=x/10;
			}
			printf("x=%d ",x);
			x=1;
			for(i=0;i<4;i++){
				l=r[i]*x+l;
				x=x*10;
			}
			n=l-s;
			a++;
		}
		if(flg==1) printf("%d\n",a);
		if(flg==0) printf("NA\n");
	}
	return 0;
}

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] = 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>
#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;
	int 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;
}