Problem p00726 (C++)

Buggy submission (status: Runtime Error):
#include <iostream>

using namespace std;

typedef long long ll;

string str;
int pos;
int loc;
ll now;

int number() {
  int res = 0;
  while (pos < str.size() && isdigit(str[pos])) {
    res = res * 10 + (int)(str[pos] - '0');
    ++pos;
  }
  return res;
}
string alp() {
  string res = "";
  while (pos < str.size() && 'A' <= str[pos] && str[pos] <= 'Z') {
    res += str[pos];
    ++pos;
  }
  return res;
}
string expr2() {
  string res = "";
  while (pos < str.size() && str[pos] != ')') {
    int n = 1;
    if (isdigit(str[pos])) {
      n = number();
    }

    string a;
    if (str[pos] == '(') {
      ++pos;
      a = expr2();
      ++pos;
    } else {
      a = alp();
    }

    for (int i = 0; i < n; ++i) {
      res += a;
    }
  }
  return res;
}
ll expr() {
  ll res = 0;
  while (pos < str.size() && str[pos] != ')') {
    int n = 1;
    if (isdigit(str[pos])) {
      n = number();
    }

    int op = pos;

    ll l;
    if (str[pos] == '(') {
      ++pos;
      l = expr();
      ++pos;
      now -= l;
    } else {
      l = alp().size();
    }

    if (now <= loc && loc < now + l * n) {
      pos = op;
      throw expr2()[(loc - now) % l];
    }

    now += l * n;
    res += l * n;
    if (now > loc + 1)
      now = loc + 1;
    if (res > loc + 1)
      res = loc + 1;
  }

  return res;
}

char parse() {
  pos = 0;

  try {
    now = 0;
    pos = 0;
    expr();
  } catch (char c) {
    return c;
  }
  return '0';
}

int main() {
  while (cin >> str >> loc, str != "0" || loc != 0) {
    cout << parse() << endl;
  }
}

Fixed submission:
#include <iostream>

using namespace std;

typedef long long ll;

string str;
int pos;
int loc;
ll now;

int number() {
  int res = 0;
  while (pos < str.size() && isdigit(str[pos])) {
    res = res * 10 + (int)(str[pos] - '0');
    ++pos;
  }
  return res;
}
string alp() {
  string res = "";
  while (pos < str.size() && 'A' <= str[pos] && str[pos] <= 'Z') {
    res += str[pos];
    ++pos;
  }
  return res;
}
string expr2() {
  string res = "";
  while (pos < str.size() && str[pos] != ')') {
    int n = 1;
    if (isdigit(str[pos])) {
      n = number();
    }

    string a;
    if (str[pos] == '(') {
      ++pos;
      a = expr2();
      ++pos;
    } else {
      a = alp();
    }

    for (int i = 0; i < n; ++i) {
      res += a;
    }
  }
  return res;
}
ll expr() {
  ll res = 0;
  while (pos < str.size() && str[pos] != ')') {
    int n = 1;
    if (isdigit(str[pos])) {
      n = number();
    }

    int op = pos;

    ll l;
    if (str[pos] == '(') {
      ++pos;
      l = expr();
      ++pos;
      now -= l;
    } else {
      l = alp().size();
    }

    if (now <= loc && loc < now + l * n) {
      pos = op;
      string s;
      if (str[pos] == '(') {
        ++pos;
        s = expr2();
        ++pos;
      } else {
        s = alp();
      }
      throw s[(loc - now) % l];
    }

    now += l * n;
    res += l * n;
    if (now > loc + 1)
      now = loc + 1;
    if (res > loc + 1)
      res = loc + 1;
  }

  return res;
}

char parse() {
  pos = 0;

  try {
    now = 0;
    pos = 0;
    expr();
  } catch (char c) {
    return c;
  }
  return '0';
}

int main() {
  while (cin >> str >> loc, str != "0" || loc != 0) {
    cout << parse() << endl;
  }
}