Problem p03128 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const ll match[10] = {inf, 2, 5, 5, 4, 5, 6, 3, 7, 6};
void update(string &a, string b) {
  if (a == "x")
    a = b;
  else if (a.size() < b.size())
    a = b;
  else if (a.size() == b.size()) {
    if (a < b)
      a = b;
  }
}
int main() {
  ll n, m;
  cin >> n >> m;
  vl a(m);
  rep(i, m) cin >> a[i];
  string dp[1010];
  rep(i, 1010) { dp[i] = "x"; }
  dp[0] = "";
  rep(i, n + 1) {
    if (dp[i] == "x")
      continue;
    for (auto p : a) {
      update(dp[i + match[p]], dp[i] + to_string(p));
    }
  }
  cout << dp[n] << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[8] = {1, 0, -1, 0, 1, 1, -1, -1};
const ll dx[8] = {0, -1, 0, 1, 1, -1, 1, -1};
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
const ll match[10] = {inf, 2, 5, 5, 4, 5, 6, 3, 7, 6};
void update(string &a, string b) {
  if (a == "x")
    a = b;
  else if (a.size() < b.size())
    a = b;
  else if (a.size() == b.size()) {
    if (a < b)
      a = b;
  }
}
int main() {
  ll n, m;
  cin >> n >> m;
  vl a(m);
  rep(i, m) cin >> a[i];
  string dp[10100];
  rep(i, 10100) { dp[i] = "x"; }
  dp[0] = "";
  rep(i, n + 1) {
    if (dp[i] == "x")
      continue;
    for (auto p : a) {
      update(dp[i + match[p]], dp[i] + to_string(p));
    }
  }
  cout << dp[n] << endl;
}