Problem p02721 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>
using namespace std;

template <typename T> istream &operator>>(istream &istr, vector<T> &v) {
  for (T &x : v)
    istr >> x;
  return istr;
}

template <typename T, typename U>
istream &operator>>(istream &istr, pair<T, U> &p) {
  istr >> p.first >> p.second;
  return istr;
}

template <typename T> ostream &operator<<(ostream &ostr, vector<T> &v) {
  if (!v.empty()) {
    ostr << v.front();
    for (auto itr = ++v.begin(); itr != v.end(); itr++)
      // ostr << *itr;
      // ostr << " " << *itr;
      ostr << endl << *itr;
  }
  return ostr;
}

template <typename T, typename U>
ostream &operator<<(ostream &ostr, pair<T, U> &p) {
  // ostr << p.first << p.second;
  ostr << p.first << ", " << p.second;
  return ostr;
}

template <typename T>
ostream &operator<<(ostream &ostr, vector<vector<T>> &vv) {
  if (!vv.empty()) {
    ostr << vv.front();
    for (auto itr = ++vv.begin(); itr != vv.end(); itr++)
      ostr << endl << *itr;
  }
  return ostr;
}

template <typename T, typename U>
ostream &operator<<(ostream &ostr, vector<pair<T, U>> &vp) {
  if (!vp.empty()) {
    ostr << vp.front();
    for (auto itr = ++vp.begin(); itr != vp.end(); itr++)
      ostr << endl << *itr;
  }
  return ostr;
}

typedef long long ll;
typedef vector<ll> vll;
typedef vector<char> vc;
typedef vector<double> vd;
// vll v(n); v[i]=x;
typedef vector<vll> vvll;
typedef vector<vc> vvc;
typedef vector<vd> vvd;
// vvll v(m,vll(n)); v[i][j]=x;
typedef pair<ll, ll> pll;
typedef pair<double, double> pd;
typedef vector<pll> vpll;
typedef vector<pd> vpd;
// vpll v(n); v[i]={x,y}; v[i].first/second;
// v.front() : v[0]
// v.back()  : v[v.size()-1]

#define PI 3.1415926535897932
#define EPS 1e-12
#define INF (ll)1e+12
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i <= (b); i++)
#define FORR(i, a, b) for (ll i = (b); i >= (a); i--)
#define fix(x) cout << fixed << setprecision(x)
#define dump(x) cout << #x << " = " << (x) << endl
#define all(x) (x).begin(), (x).end()

template <typename T> T sum(vector<T> &v) { return accumulate(all(v), (T)0); }
template <typename T> inline void sort(vector<T> &v) { sort(all(v)); }
template <typename T> inline void rsort(vector<T> &v) {
  sort(all(v), greater<T>());
}
template <typename T> inline void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <typename T> inline void chmax(T &a, T b) {
  if (a < b)
    a = b;
}

int main() {
  ll n, k, c;
  cin >> n >> k >> c;
  vc s(n);
  cin >> s;

  ll p = 0, q = n - 1;
  vll l, r, v;
  REP(i, k) {
    while (s[p] == 'x')
      p++;
    l.emplace_back(p + 1);
    p += c + 1;
    while (s[q] == 'x')
      q--;
    r.emplace_back(q + 1);
    q -= c + 1;
  }
  sort(r);
  // set_intersection(all(l), all(r), back_inserter(v));
  REP(i, k) if (l[i] == r[i]) cout << l[i] << endl;
  return 0;
}


Error: WA

Fixed submission:

#include <bits/stdc++.h>
using namespace std;

template <typename T> istream &operator>>(istream &istr, vector<T> &v) {
  for (T &x : v)
    istr >> x;
  return istr;
}

template <typename T, typename U>
istream &operator>>(istream &istr, pair<T, U> &p) {
  istr >> p.first >> p.second;
  return istr;
}

template <typename T> ostream &operator<<(ostream &ostr, vector<T> &v) {
  if (!v.empty()) {
    ostr << v.front();
    for (auto itr = ++v.begin(); itr != v.end(); itr++)
      // ostr << *itr;
      ostr << " " << *itr;
  }
  return ostr;
}

template <typename T, typename U>
ostream &operator<<(ostream &ostr, pair<T, U> &p) {
  // ostr << p.first << p.second;
  ostr << p.first << ", " << p.second;
  return ostr;
}

template <typename T>
ostream &operator<<(ostream &ostr, vector<vector<T>> &vv) {
  if (!vv.empty()) {
    ostr << vv.front();
    for (auto itr = ++vv.begin(); itr != vv.end(); itr++)
      ostr << endl << *itr;
  }
  return ostr;
}

template <typename T, typename U>
ostream &operator<<(ostream &ostr, vector<pair<T, U>> &vp) {
  if (!vp.empty()) {
    ostr << vp.front();
    for (auto itr = ++vp.begin(); itr != vp.end(); itr++)
      ostr << endl << *itr;
  }
  return ostr;
}

typedef long long ll;
typedef vector<ll> vll;
typedef vector<char> vc;
typedef vector<double> vd;
// vll x(n); x[i]=a;
typedef vector<vll> vvll;
typedef vector<vc> vvc;
typedef vector<vd> vvd;
// vvll x(m,vi(n)); x[i][j]=a;
typedef pair<ll, ll> pll;
typedef pair<double, double> pd;
typedef vector<pll> vpll;
typedef vector<pd> vpd;
// vpll x(n); x[i]={a,b}; x[i].first/second;

#define PI 3.1415926535897932
#define EPS 1e-12
#define INF (ll)1e+12
#define REP(i, n) for (ll i = 0; i < (n); i++)
#define RREP(i, n) for (ll i = (n)-1; i >= 0; i--)
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define FORR(i, a, b) for (ll i = (b)-1; i >= (a); i--)
#define fix(x) cout << fixed << setprecision(x)
#define dump(x) cout << #x << " = " << (x) << endl
#define all(x) (x).begin(), (x).end()

template <typename T> T sum(vector<T> &v) { return accumulate(all(v), (T)0); }
template <typename T> inline void sort(vector<T> &v) { sort(all(v)); }
template <typename T> inline void rsort(vector<T> &v) {
  sort(all(v), greater<T>());
}
template <typename T> inline void chmin(T &a, T b) {
  if (a > b)
    a = b;
}
template <typename T> inline void chmax(T &a, T b) {
  if (a < b)
    a = b;
}

int main() {
  ll n, k, c;
  cin >> n >> k >> c;
  vc s(n);
  cin >> s;

  ll p = 0, q = n - 1;
  vll l, r;
  REP(i, k) {
    while (s[p] == 'x')
      p++;
    l.emplace_back(p + 1);
    p += c + 1;
    while (s[q] == 'x')
      q--;
    r.emplace_back(q + 1);
    q -= c + 1;
  }

  sort(r);
  p = 0;
  q = 0;

  REP(i, 2 * n) {
    if (l[p] == r[q])
      cout << l[p] << endl;
    if (l[p] <= r[q])
      p++;
    else
      q++;
  }
  return 0;
}