Problem:
Score : 400 points

Problem Statement

Find the largest integer that can be formed with exactly N matchsticks, under the following conditions:

Every digit in the integer must be one of the digits A_1, A_2, ..., A_M (1 \leq A_i \leq 9).

The number of matchsticks used to form digits 1, 2, 3, 4, 5, 6, 7, 8, 9 should be 2, 5, 5, 4, 5, 6, 3, 7, 6, respectively.

Constraints

All values in input are integers.

2 \leq N \leq 10^4

1 \leq M \leq 9

1 \leq A_i \leq 9

A_i are all different.

There exists an integer that can be formed by exactly N matchsticks under the conditions.

Input

Input is given from Standard Input in the following format:

N M
A_1 A_2 ... A_M

Output

Print the largest integer that can be formed with exactly N matchsticks under the conditions in the problem statement.

Sample Input 1

20 4
3 7 8 4

Sample Output 1

777773

The integer 777773 can be formed with 3 + 3 + 3 + 3 + 3 + 5 = 20 matchsticks, and this is the largest integer that can be formed by 20 matchsticks under the conditions.

Sample Input 2

101 9
9 8 7 6 5 4 3 2 1

Sample Output 2

71111111111111111111111111111111111111111111111111

The output may not fit into a 64-bit integer type.

Sample Input 3

15 3
5 4 6

Sample Output 3

654

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var N = sc.ReadInt();
        var M = sc.ReadInt();
        var A = sc.ReadIntArray(M);
        Array.Sort(A);
        Array.Reverse(A);
        var Match = new int[] { 0, 2, 5, 5, 4, 5, 6, 3, 7, 6 };

        var dp = new string[N + 1];
        dp[0] = "";
        for (int i = 1; i <= N; i++)
        {
            string now = "";
            foreach (var a in A)
            {
                var match = Match[a];
                if ((i - match > 0 && dp[i - match] != "") || i - match == 0)
                {
                    if (dp[i - match] == "" || dp[i - match][0] <= a.ToString()[0])
                    {
                        var next = a.ToString() + dp[i - match];
                        if (now.Length < next.Length) {
                            now = next;
                        }
                    }
                }
            }
            dp[i] = now;
        }
        var s = dp[N].ToArray();
        Array.Sort(s);
        Array.Reverse(s);
        var ans = new string(s);
        Console.WriteLine(ans);
    }

    static long MOD = 1000000000 + 7;
    static int INF = (int)1e7 + 1;
    static long LINF = (long)1e13 + 1;
    static Random Rand = new Random();
    static Scanner sc = new Scanner();

    static long Gcd(long a, long b)
    {
        var v = new[] { a, b };
        while (v[1] != 0) { v = new[] { v[1], v[0] % v[1] }; }
        return v[0];
    }
    static long Lcm(long a, long b)
    {
        return a / Gcd(a, b) * b;
    }
    static bool IsPrime(long num)
    {
        if (num < 2) return false;
        else if (num == 2) return true;
        else if (num % 2 == 0) return false;

        double sqrtNum = Math.Sqrt(num);
        for (int i = 3; i <= sqrtNum; i += 2)
        {
            if (num % i == 0)
            {
                return false;
            }
        }
        return true;
    }
    static List<long> PrimeFactors(long n)
    {
        var res = new List<long>();
        var tmp = n;
        for (long x = 2; x * x <= n; x++)
        {
            while (tmp % x == 0)
            {
                tmp /= x;
                res.Add(x);
            }
        }
        if (tmp != 1) res.Add(tmp);
        return res;
    }
    static long[] Divisor(long n)
    {
        var res = new List<long>();
        for (int i = 1; i * i <= n; i++)
        {
            if (n % i == 0)
            {
                res.Add(i);
                if (i * i != n) res.Add(n / i);
            }
        }
        return res.ToArray();
    }
    static Result[] ListAll<T1, T2, Result>(IEnumerable<T1> a, IEnumerable<T2> b, Func<T1, T2, Result> f)
    {
        Result[] res = new Result[a.Count() * b.Count()];
        var i = 0;
        foreach (var v in a)
            foreach (var w in b)
                res[i++] = f(v, w);
        return res;
    }
    static IEnumerable<Result> ListAll<T1, T2, Result>(IEnumerable<T1> a, IEnumerable<T2> b, Func<T1, T2, Result> f, Func<T1, T2, bool> s)
    {
        foreach (var v in a)
        {
            foreach (var w in b)
                if (s(v, w)) yield return f(v, w);
        }
    }
    static List<long> Facs = new List<long>() { 1 };
    static long Fac(int n)
    {
        for (int i = Facs.Count; i <= n; i++)
        {
            Facs.Add(i * Facs[i - 1]);
        }
        return Facs[n];
    }
    static long nCr(int n, int r)
    {
        if (n < r) return 0;
        if (n == r) return 1;
        var res = Fac(n);
        res /= Fac(r);
        res /= Fac(n - r);
        return res;
    }
}

class Modulo
{
    public readonly int MOD;
    private readonly List<int> Facs = new List<int>() { 1 };

    public Modulo(int mod = 1000000007) { this.MOD = mod; }
    public int Mul(int a, int b) { return (int)(Math.BigMul(a, b) % MOD); }
    public int Div(int a, int b) { return Mul(a, Pow(b, MOD - 2)); }
    public int Pow(int a, int n)
    {
        switch (n)
        {
            case 0:
                return 1;
            case 1:
                return a;
            default:
                int p1 = Pow(a, n / 2);
                int p2 = Mul(p1, p1);
                return ((n % 2) == 0) ? p2 : Mul(p2, a);
        }
    }
    public int Fac(int n)
    {
        for (int i = this.Facs.Count; i <= n; i++)
        {
            this.Facs.Add(Mul(i, Facs[i - 1]));
        }
        return this.Facs[n];
    }
    public int nPr(int n, int r) { return Div(Fac(n), Fac(n - r)); }
    public int nCr(int n, int r)
    {
        if (n < r) return 0;
        if (n == r) return 1;
        int res = Fac(n);
        res = Div(res, Fac(r));
        res = Div(res, Fac(n - r));
        return res;
    }
}

class Debug
{
    public static void Dump<T>(T[,] array)
    {
        Console.WriteLine();
        var h = array.GetLength(0);
        var w = array.GetLength(1);

        for (int r = 0; r < h; r++)
        {
            for (int c = 0; c < w; c++)
            {
                Console.Write(array[r, c]);
                Console.Write(" ");
            }
            Console.WriteLine();
        }
        Console.WriteLine();
    }
    public static void Dump<T>(T[] array)
    {
        Console.WriteLine();
        Console.WriteLine(string.Join(" ", array));
        Console.WriteLine();
    }
}

class Scanner
{
    string[] S = new string[0];
    int Index = 0;
    char[] Separators = new char[] { ' ' };

    public string Next()
    {
        if (this.Index < this.S.Length) return this.S[this.Index++];
        var line = "";
        while (line == "") line = Console.ReadLine();
        this.S = line.Split(this.Separators, StringSplitOptions.RemoveEmptyEntries);
        if (this.S.Length == 0) return this.Next();
        this.Index = 0;
        return this.S[this.Index++];
    }

    public string ReadStr()
    {
        return this.Next();
    }

    public char ReadChar()
    {
        return this.Next()[0];
    }

    public int ReadInt()
    {
        return int.Parse(this.Next());
    }

    public uint ReadUInt()
    {
        return uint.Parse(this.Next());
    }

    public long ReadLong()
    {
        return long.Parse(this.Next());
    }

    public int[] ReadIntArray(int n)
    {
        var array = new int[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadInt();
        }
        return array;
    }

    public long[] ReadLongArray(int n)
    {
        var array = new long[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadLong();
        }
        return array;
    }

    public char[] ReadCharArray(int n)
    {
        var array = new char[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadChar();
        }
        return array;
    }

    public string[] ReadStrArray(int n)
    {
        var array = new string[n];
        for (int i = 0; i < array.Length; i++)
        {
            array[i] = this.ReadStr();
        }
        return array;
    }
}

static class Permutation<T>
{
    private static void Search(List<T[]> perms, Stack<T> stack, T[] a)
    {
        int N = a.Length;
        if (N == 0)
        {
            perms.Add(stack.Reverse().ToArray());
        }
        else
        {
            var b = new T[N - 1];
            Array.Copy(a, 1, b, 0, N - 1);
            for (int i = 0; i < a.Length; ++i)
            {
                stack.Push(a[i]);
                Search(perms, stack, b);
                if (i < b.Length) { b[i] = a[i]; }
                stack.Pop();
            }
        }
    }
    public static List<T[]> All(params T[] source)
    {
        var perms = new List<T[]>();
        Search(perms, new Stack<T>(), source);
        return perms;
    }
}

class HashMap<K, V> : Dictionary<K, V> where V : new()
{
    private V DefaltValue = new V();
    public HashMap() { }
    public HashMap(V defaultValue)
    {
        this.DefaltValue = new V();
    }
    new public V this[K i]
    {
        get
        {
            V v;
            return TryGetValue(i, out v) ? v : base[i] = this.DefaltValue;
        }
        set { base[i] = value; }
    }
}

class Vector<T> : List<T> where T : new()
{
    public Vector() { }
    public Vector(int count, T defaultValue)
    {
        for (int i = 0; i < count; i++)
        {
            this.Add(defaultValue);
        }
    }
}

class PriorityQueue<T> where T : IComparable<T>
{
    public readonly T[] Array;
    public readonly bool Greater;
    public int Count { get; private set; } = 0;

    public PriorityQueue(int capacity, bool greater = true)
    {
        this.Array = new T[capacity];
        this.Greater = greater;
    }

    public void Enqueue(T item)
    {
        this.Array[this.Count] = item;
        this.Count += 1;
        this.UpHeap();
    }

    public T Dequeue()
    {
        this.Swap(0, this.Count - 1);
        this.Count -= 1;
        this.DownHeap();

        return this.Array[this.Count];
    }

    public T Peek()
    {
        return this.Array[0];
    }

    private void UpHeap()
    {
        var n = this.Count - 1;
        while (n != 0)
        {
            var parent = (n - 1) / 2;

            if (this.Array[n].CompareTo(this.Array[parent]) > 0)
            {
                this.Swap(n, parent);
                n = parent;
            }
            else
            {
                break;
            }
        }
    }

    private void DownHeap()
    {
        var parent = 0;
        while (true)
        {
            var child = 2 * parent + 1;
            if (child > this.Count - 1) break;

            if (child < this.Count - 1 && this.Compare(this.Array[child], this.Array[child + 1]) < 0)
            {
                child += 1;
            }

            if (this.Compare(this.Array[parent], this.Array[child]) < 0)
            {
                this.Swap(parent, child);
                parent = child;
            }
            else
            {
                break;
            }
        }
    }

    private int Compare(T a, T b)
    {
        var c = a.CompareTo(b);
        if (!this.Greater)
        {
            c = -c;
        }
        return c;
    }

    private void Swap(int a, int b)
    {
        var tmp = this.Array[a];
        this.Array[a] = this.Array[b];
        this.Array[b] = tmp;
    }
}

class UnionFind
{
    public int[] Parents { get; set; }
    public UnionFind(int n)
    {
        this.Parents = new int[n];
        for (int i = 0; i < n; i++)
        {
            this.Parents[i] = i;
        }
    }
    public int Find(int x)
    {
        if (this.Parents[x] != x)
        {
            this.Parents[x] = Find(this.Parents[x]);
        }
        return this.Parents[x];
    }
    public bool Same(int x, int y)
    {
        return this.Find(x) == this.Find(y);
    }
    public int Union(int x, int y)
    {
        var xroot = this.Find(x);
        var yroot = this.Find(y);
        if (xroot != yroot) this.Parents[xroot] = yroot;
        return yroot;
    }
}
static class Algorithm
{
    static int binarySearch<T>(IList<T> a, T v, IComparer<T> cmp, bool islb)
    {
        var l = 0;
        var r = a.Count - 1;
        while (l <= r)
        {
            var m = (l + r) / 2;
            var res = cmp.Compare(a[m], v);
            if (res < 0 || (res == 0 && !islb)) l = m + 1;
            else r = m - 1;
        }
        return l;
    }
    public static int LowerBound<T>(this IList<T> a, T v, Comparison<T> f) { return binarySearch(a, v, Comparer<T>.Create(f), true); }
    public static int LowerBound<T>(this IList<T> a, T v) { return binarySearch(a, v, Comparer<T>.Default, true); }
    public static int UpperBound<T>(this IList<T> a, T v, Comparison<T> cmp) { return binarySearch(a, v, Comparer<T>.Create(cmp), false); }
    public static int UpperBound<T>(this IList<T> a, T v) { return binarySearch(a, v, Comparer<T>.Default, false); }
}