Problem p00736 (C++) — buggy submission, verdict: Runtime Error

#include <bits/stdc++.h>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i, c)                                                             \\
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define exist(s, e) ((s).find(e) != (s).end())
#define range(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) range(i, 0, n)
#define clr(a, b) memset((a), (b), sizeof(a))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \\
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \\
       << " " << __FILE__ << endl;

const double eps = 1e-10;
const double pi = acos(-1.0);
const ll INF = 1LL << 62;
const int inf = 1 << 24;

int mns(int val) {
  if (val == 0)
    val = 2;
  else if (val == 2)
    val = 0;

  return val;
}

int mlt(int lhs, int rhs) { return min(lhs, rhs); }

int pls(int lhs, int rhs) { return max(lhs, rhs); }

int parse(string &str, int &pos) {
  int ret;

  if (str.at(pos) == '(') {
    pos++;

    int lhs = parse(str, pos);
    char op = str.at(pos);
    pos++;
    int rhs = parse(str, pos);

    switch (op) {
    case '+': {
      ret = pls(lhs, rhs);
      break;
    }
    case '*': {
      ret = mlt(lhs, rhs);
      break;
    }
    default:
      break;
    }

    pos++;
  } else if (str.at(pos) == '-') {
    int cnt = 0;
    while (str.at(pos) == '-') {
      cnt++;
      pos++;
    }

    ret = parse(str, pos);
    if (cnt % 2 == 1)
      ret = mns(ret);
  } else {
    ret = str.at(pos) - '0';
    pos++;
  }

  return ret;
}

int main(void) {
  for (string str_org; cin >> str_org, str_org != ".";) {
    int res = 0;

    rep(p, 3) {
      rep(q, 3) {
        rep(r, 3) {
          string str = str_org;

          for (auto &&c : str) {
            switch (c) {
            case 'P': {
              c = p + '0';
              break;
            }
            case 'Q': {
              c = q + '0';
              break;
            }
            case 'R': {
              c = r + '0';
              break;
            }
            default:
              break;
            }
          }

          int pos = 0;
          if (parse(str, pos) == 2) {
            res++;
          }
        }
      }
    }

    cout << res << endl;
  }

  return 0;
}

Error: -6
terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::at: __n (which is 0) >= this->size() (which is 0)


Fixed submission:

#include <algorithm>
#include <bitset>
#include <cctype>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>

using namespace std;

inline int toInt(string s) {
  int v;
  istringstream sin(s);
  sin >> v;
  return v;
}
template <class T> inline string toString(T x) {
  ostringstream sout;
  sout << x;
  return sout.str();
}
template <class T> inline T sqr(T x) { return x * x; }

typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<string> vs;
typedef pair<int, int> pii;
typedef long long ll;

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair
#define each(i, c)                                                             \\
  for (typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define exist(s, e) ((s).find(e) != (s).end())
#define range(i, a, b) for (int i = (a); i < (b); ++i)
#define rep(i, n) range(i, 0, n)
#define clr(a, b) memset((a), (b), sizeof(a))
#define dump(x) cerr << #x << " = " << (x) << endl;
#define debug(x)                                                               \\
  cerr << #x << " = " << (x) << " (L" << __LINE__ << ")"                       \\
       << " " << __FILE__ << endl;

const double eps = 1e-10;
const double pi = acos(-1.0);
const ll INF = 1LL << 62;
const int inf = 1 << 29;

int pos;
const string kVars = "abcdefghijk";
string formula1, formula2, res;

int mlt(int lhs, int rhs) { return lhs && rhs; }

int pls(int lhs, int rhs) { return lhs || rhs; }

int mns(int val) { return !val; }

int imp(int lhs, int rhs) { return !(lhs && !rhs); }

int operate(const string &formula, int &pos) {
  int ret;

  if (formula.at(pos) == '(') {
    pos++;

    int lhs = operate(formula, pos);
    char op = formula.at(pos++);
    if (op == '-')
      pos++;
    int rhs = operate(formula, pos);

    switch (op) {
    case '*': {
      ret = mlt(lhs, rhs);
      break;
    }
    case '+': {
      ret = pls(lhs, rhs);
      break;
    }
    case '-': {
      ret = imp(lhs, rhs);
      break;
    }
    default:
      break;
    }

    pos++;
  }

  else if (formula.at(pos) == '-') {
    bool isMns = true;
    while (formula.at(++pos) == '-')
      isMns = !isMns;

    ret = operate(formula, pos);
    if (isMns)
      ret = mns(ret);
  }

  else {
    ret = formula.at(pos) - '0';

    pos++;
  }

  return ret;
}

void dfs(int p) {
  if (p == kVars.size()) {
    if (operate(formula1, pos = 0) != operate(formula2, pos = 0)) {
      res = "NO";
    }

    return;
  }

  rep(i, 2) {
    string tmp1 = formula1, tmp2 = formula2;

    replace(all(formula1), kVars.at(p), (char)(i + '0'));
    replace(all(formula2), kVars.at(p), (char)(i + '0'));

    dfs(p + 1);

    formula1 = tmp1, formula2 = tmp2;
  }
}

int main(void) {
  for (string in; cin >> in, in != "#"; cout << res << endl) {
    res = "YES";

    replace(all(in), 'T', '1');
    replace(all(in), 'F', '0');
    replace(all(in), '=', ' ');

    stringstream ss(in);
    ss >> formula1;
    ss >> formula2;

    dfs(0);
  }

  return 0;
}