Problem p03286 (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 mp make_pair
#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 at(x, i) get<i>(x);
#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;
}
int main() {
  ll n;
  cin >> n;
  vl r(30), l(30);
  ll left = 0, right = 0;
  ll cnt = 1;
  rep(i, 50) {
    if (cnt > 0)
      right += cnt;
    else
      left += cnt;
    r[i] = right;
    l[i] = left;
    cnt *= -2;
  }
  cnt = -cnt / 2;
  string ans;
  bool check = false;
  for (int i = 49; i >= 1; i--) {
    if (l[i - 1] <= n && r[i - 1] >= n) {
      if (check)
        ans += '0';
    } else {
      n -= cnt;
      ans += '1';
      check = true;
    }
    cnt = -cnt / 2;
  }
  if (n == 1)
    ans += '1';
  if (n == 0)
    ans += '0';
  cout << ans << 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 mp make_pair
#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 at(x, i) get<i>(x);
#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;
}
int main() {
  ll n;
  cin >> n;
  vl r(50), l(50);
  ll left = 0, right = 0;
  ll cnt = 1;
  rep(i, 50) {
    if (cnt > 0)
      right += cnt;
    else
      left += cnt;
    r[i] = right;
    l[i] = left;
    cnt *= -2;
  }
  cnt = -cnt / 2;
  string ans;
  bool check = false;
  for (int i = 49; i >= 1; i--) {
    if (l[i - 1] <= n && r[i - 1] >= n) {
      if (check)
        ans += '0';
    } else {
      n -= cnt;
      ans += '1';
      check = true;
    }
    cnt = -cnt / 2;
  }
  if (n == 1)
    ans += '1';
  if (n == 0)
    ans += '0';
  cout << ans << endl;
}