Problem p03696 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

#define MOD 1000000007
#define MOD2 998244353
#define int long long
#define EPS 1e-9
// #define PI 3.14159265358979

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &A) {
  for (int i = 0; i < A.size(); i++)
    os << A[i] << " ";
  os << endl;
  return os;
}
template <> ostream &operator<<(ostream &os, const vector<vector<int>> &A) {
  int N = A.size();
  int M;
  if (N > 0)
    M = A[0].size();
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < M; j++)
      os << A[i][j] << " ";
    os << endl;
  }
  return os;
}

typedef pair<int, int> pii;
typedef long long ll;

struct edge {
  int from, to, d, c;
  edge(int _from = 0, int _to = 0, int _d = 0, int _c = 0) {
    from = _from;
    to = _to;
    d = _d;
    c = _c;
  }
  bool operator<(const edge &rhs) const {
    return (d == rhs.d) ? (c < rhs.c) : (d < rhs.d);
  }
};
struct aabb {
  int x1, y1, x2, y2;
  aabb(int x1, int y1, int x2, int y2) : x1(x1), y1(y1), x2(x2), y2(y2) {}
};

typedef vector<edge> edges;
typedef vector<edges> graph;
struct flow {
  int to, cap, rev, cost;
  flow(int to = 0, int cap = 0, int rev = 0, int cost = 0)
      : to(to), cap(cap), rev(rev), cost(cost) {}
};
typedef vector<vector<flow>> flows;

const int di[4] = {0, -1, 0, 1};
const int dj[4] = {-1, 0, 1, 0};
const int ci[5] = {0, 0, -1, 0, 1};
const int cj[5] = {0, -1, 0, 1, 0};
const ll LINF = LLONG_MAX / 2;
const int INF = INT_MAX / 2;
const double PI = acos(-1);

int pow2(int n) { return 1LL << n; }
template <typename T, typename U> bool chmin(T &x, const U &y) {
  if (x > y) {
    x = y;
    return true;
  }
  return false;
}
template <typename T, typename U> bool chmax(T &x, const U &y) {
  if (x < y) {
    x = y;
    return true;
  }
  return false;
}

struct initializer {
  initializer() { cout << fixed << setprecision(20); }
};
initializer _____;

int N, M, K, T, Q, H, W;
signed main() {
  cin >> N;
  string S;
  cin >> S;
  int l = 0, r = 0;
  rep(i, N) {
    if (S[i] == '(')
      ++l;
    else if (l > 0)
      --l;
  }
  reverse(S.begin(), S.end());
  rep(i, N) {
    if (S[i] == ')')
      ++r;
    else if (r > 0)
      --l;
  }
  reverse(S.begin(), S.end());
  cout << string(r, '(') << S << string(l, ')');
  return 0;
}

Fixed submission:
#include <algorithm>
#include <bitset>
#include <climits>
#include <cmath>
#include <cstdio>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <vector>

#define MOD 1000000007
#define MOD2 998244353
#define int long long
#define EPS 1e-9
// #define PI 3.14159265358979

#define rep(i, n) for (int i = 0; i < (int)(n); i++)

using namespace std;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &A) {
  for (int i = 0; i < A.size(); i++)
    os << A[i] << " ";
  os << endl;
  return os;
}
template <> ostream &operator<<(ostream &os, const vector<vector<int>> &A) {
  int N = A.size();
  int M;
  if (N > 0)
    M = A[0].size();
  for (int i = 0; i < N; i++) {
    for (int j = 0; j < M; j++)
      os << A[i][j] << " ";
    os << endl;
  }
  return os;
}

typedef pair<int, int> pii;
typedef long long ll;

struct edge {
  int from, to, d, c;
  edge(int _from = 0, int _to = 0, int _d = 0, int _c = 0) {
    from = _from;
    to = _to;
    d = _d;
    c = _c;
  }
  bool operator<(const edge &rhs) const {
    return (d == rhs.d) ? (c < rhs.c) : (d < rhs.d);
  }
};
struct aabb {
  int x1, y1, x2, y2;
  aabb(int x1, int y1, int x2, int y2) : x1(x1), y1(y1), x2(x2), y2(y2) {}
};

typedef vector<edge> edges;
typedef vector<edges> graph;
struct flow {
  int to, cap, rev, cost;
  flow(int to = 0, int cap = 0, int rev = 0, int cost = 0)
      : to(to), cap(cap), rev(rev), cost(cost) {}
};
typedef vector<vector<flow>> flows;

const int di[4] = {0, -1, 0, 1};
const int dj[4] = {-1, 0, 1, 0};
const int ci[5] = {0, 0, -1, 0, 1};
const int cj[5] = {0, -1, 0, 1, 0};
const ll LINF = LLONG_MAX / 2;
const int INF = INT_MAX / 2;
const double PI = acos(-1);

int pow2(int n) { return 1LL << n; }
template <typename T, typename U> bool chmin(T &x, const U &y) {
  if (x > y) {
    x = y;
    return true;
  }
  return false;
}
template <typename T, typename U> bool chmax(T &x, const U &y) {
  if (x < y) {
    x = y;
    return true;
  }
  return false;
}

struct initializer {
  initializer() { cout << fixed << setprecision(20); }
};
initializer _____;

int N, M, K, T, Q, H, W;
signed main() {
  cin >> N;
  string S;
  cin >> S;
  int l = 0, r = 0;
  rep(i, N) {
    if (S[i] == '(')
      ++l;
    else if (l > 0)
      --l;
  }
  reverse(S.begin(), S.end());
  rep(i, N) {
    if (S[i] == ')')
      ++r;
    else if (r > 0)
      --r;
  }
  reverse(S.begin(), S.end());
  cout << string(r, '(') << S << string(l, ')');
  return 0;
}