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]
	return solve(l-s) + 1
}

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 <bits/stdc++.h>
using namespace std;

int main()
{
    string N;
    while ( cin >> N, N != "0000" ) {
        int cnt = 0;
        set < string > st;

        while ( N != "6174" && st.find(N) == st.end() ) {
            string L = N, S = N;
            int val = 0;
            st.insert(N);
            sort(rbegin(L), rend(L));
            sort(begin(S), end(S));
            val = atoi(L.c_str()) - atoi(S.c_str());
            N = to_string(val);
            while ( N.size() < 4 ) {
                N = '0' + N;
            }
            cnt++;
        }

        if ( N == "6174" ) {
            cout << cnt << endl;
        } else {
            cout << "NA" << endl;
        }
    }

    return ( 0 );
}

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

bool isSameNumber(vector<int> n){
    bool isSame = true;
    for(int i=0; i < n.size(); i++){
        for(int j=0; j < n.size(); j++){
            if(n[i] != n[j])isSame = false;
        }
    }
    return isSame;
}

int main(){
    vector<int> n;
    int input;
    while(cin >> input){
        int ans = 0;
        bool isok = true;
        if(input == 0) break;
        if(input % 1111 == 0){
            cout << "NA" << endl;
            continue;
        }
        while(input != 6174) {
            if(log10(input) < 3) n.push_back(0);
            while(input != 0){
                n.push_back(input%10);
                input /= 10;
            }
            vector<int> minNum ,maxNum;

            sort(n.begin(), n.end());
            minNum = n;
            reverse(n.begin(), n.end());
            maxNum = n;

            int p=0, q=0;
            for(int i=0, m = 1000; i < 4; i++, m/=10){
                p += minNum[i]*m;
                q += maxNum[i]*m;
            }
            input = q-p;
            ans++;

            n.clear();
        }
        if(isok)cout << ans << endl;
    }
}

C++: #include <iostream>
#include <algorithm> 
using namespace std;
int main(){
	int N; 
	int n[4];
	while(true){
		int cnt=0;
		cin >> N;
		if(N/1000==0) break;
		if(N%1111==0) {
			cout << "NA\n";
			continue;
		}
		while(N!=6174){
			n[0]=N/1000;
			n[1]=(N%1000)/100;
			n[2]=((N%1000)%100)/10;
			n[3]=((N%1000)%100)%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 <iostream>
#include <cstdio>
#include <cstdlib>
#include <time.h>
#include <math.h>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <algorithm>
#include <iterator>
#include <sstream>
#include <string>
#include <bitset>
using namespace std;
#define FOR(I,F,N) for(int I = F; I < (int)(N); I++)
#define rep(i, n) FOR(i, 0, n)
#define FIN(V) cout<<V<<endl
#define pb push_back
#define INF (1 << 30)
typedef pair<int, int> P;
typedef long long ll;
typedef priority_queue<int> pq;
int StrToInt(string);
string IntToStr(int);
int dx[4] = {1, -1, 0, 0};
int dy[4] = {0, 0, 1, -1};

int main(void){
	string S;
	while(cin >> S){
		int cost = 0;
		string str = S;
		if(S == "6174"){
			cout << cost << endl;
			continue;
		}
		if(S == "0000")break;
		if(StrToInt(S) % 1111 == 0){
			cout << "NA" << endl;
			continue;
		}
		while(1){
			cost++;
			vector<int> v;
			rep(i,4){
				int a = str[i]-'0';
				v.push_back(a);
			}
			sort(v.begin(), v.end(), greater<int>());
			int L = v[0] * 1000 + v[1] * 100 + v[2] * 10 + v[3];
			sort(v.begin(), v.end());
			int S = v[0] * 1000 + v[1] * 100 + v[2] * 10 + v[3];
			int sum = L - S;
			cout << "L:" << L << " S:" << S << endl;
			cout << "cost:" << cost << " " << sum << endl;
			if(sum == 6174){
				cout << cost << endl;
				break;
			}
			str = IntToStr(sum);
		}


	}
	return 0;
}



int StrToInt(string s){
	stringstream ss;
	ss << s;
	int val;
	ss >> val;
	return val;
}
string IntToStr(int i){
	stringstream ss;
	ss << i;
	return ss.str();
}

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;
					}
				}
			}
			x=1000;
			for(i=0;i<4;i++){
				s=r[i]*x+s;
				x=x/10;
			}
			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 <iostream>
#include <string>
#include <vector>
#include <cstdio>
#include <algorithm>
using namespace std;

int check(int a){
  vector<int> b;
  for(int i=0; i<4; i++){
    b.push_back(a%10);
    a/=10;
  }
  sort(b.begin(),b.end());
  int p=0, q=0, u=1;
  for(int i=0; i<4; i++){
    q+=b[i]*u;
    p+=b[3-i]*u;
    u*=10;
  }
  return q-p;
}

int main(){
  int t;
  while(true){
    cin >> t;
    if(t==0)
      break;
    bool f = true;
    for(int i=1; i<9; i++)
      if(t==i*1111){
	cout << "NA\n";
	f = false;
      }
    if(f){
      if(t==6174)
	cout << 0 << endl;
      else{
	int kk = 1;
	int pp = check(t);
	while(pp != 6174){
	  pp = check(pp);
	  kk++;
	}
	cout << kk << endl;   
      }   
    }
  }
  return 0;
}

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

char N[4], n[4];
int L, S, d;

int main()
{
  while(true)
  {
    cin >> N;
    if(N[0] == '0' && N[1] == '0' && N[2] == '0' && N[3] == '0')
      break;
    if(N[0] == N[1] && N[1] == N[2] && N[2] == N[3])
    {
      cout << "NA/n";
      continue;
    }
    for(int i = 0; i < 10; i++)
    {
      if(N[0] == '6' && N[1] == '1' && N[2] == '7' && N[3] == '4')
      {
        cout << i << endl;
        break;
      }

      L = S = 0;
      sort(N, N + 4);
      for(int j = 0; j < 4; j++)
        S += (N[j] - '0') * pow(10, 3 - j);
      reverse(N , N + 4);
      for(int j = 0; j < 4; j++)
        L += (N[j] - '0') * pow(10, 3 - j);
      d = L - S;
      for(int j = 0; j < 4; j++)
      {
        N[j] = (d / (int)pow(10, 3 - j)) + '0';
        d -= (N[j] - '0') * (int)pow(10, 3 - j);
      }
    }
  }
}