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 <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;
}

C++: #include <iostream>
#include <queue>
#include <vector>
#include <algorithm>
#include <vector>

std::string sub(const std::string& a, const std::string& b) {
    int a_ = 0, b_ = 0;

    int k = 1;
    for(int i = 3; i >= 0; --i) {
        a_ += k * (a[i] - '0');
        b_ += k * (b[i] - '0');

        k *= 10;
    }

    return std::to_string(a_ - b_);
}

int func(int i, const std::string& s) {
    if(s == "6174") {
        return i;
    }

    auto a = s, b = s;

    std::sort(a.begin(), a.end());
    std::sort(b.begin(), b.end(), std::greater<char>());

    return func(i + 1, sub(b, a));
}

inline
bool same(const std::string& a) {
    bool same = true;

    for(int i = 1; i < a.size(); ++i) {
        if(a[i - 1] != a[i]) {
            same = false;
        }
    }

    return same;
}

int main() {
    std::string  n;

    while(std::cin>> n, n != "0000") {
        if(same(n)) {
            std::cout << "NA" << std::endl;

            continue;
        }

        std::cout << func(0, n) << std::endl;
    }
}

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;
    int ans = 0;
    while(cin >> input){
        bool isok = true;
        if(log10(input) < 3) n.push_back(0);
        if(input == 0) break;
        while(input != 6174) {
            while(input != 0){
                n.push_back(input%10);
                input /= 10;
            }
            if(isSameNumber(n)){
                cout << "NA" << endl;
                n.clear();
                isok = false;
                break;
            }
            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;
    }
}