Problem p00925 (C++) — buggy submission, verdict: Runtime Error

#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for (int(i) = (a)-1; (i) >= 0; (i)--)
#define FORR(i, a, b) for (int(i) = (a)-1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template <typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

vector<string> split(const string &originalStr, char sep) {
  vector<string> res;
  string element = "";
  for (const char c : originalStr) {
    if (c == sep) {
      res.emplace_back(element);
      element.clear();
    } else {
      element += c;
    }
  }
  if (!element.empty()) {
    res.emplace_back(element);
  }
  return res;
}

char strbuf[1000];
string S;
int ans;

int leftToRightCalc(string s) {
  const int L = s.length();
  if (L == 0)
    return 0;
  int res = s[0] - '0';
  bool premul = false;
  FOR(i, 1, L) {
    if (isdigit(s[i])) {
      if (premul) {
        res *= s[i] - '0';
      } else {
        res += s[i] - '0';
      }
    } else {
      premul = s[i] == '*';
    }
  }
  return res;
}
int mul(string s) {
  if (s.length() == 0)
    return 0;
  int res = s[0] - '0';
  FOR(i, 1, s.length()) {
    if (isdigit(s[i])) {
      res *= s[i] - '0';
    }
  }
  // DEBUG(res)
  return res;
}
int mulFirstCalc(string s) {
  VT<string> S = split(s, '+');
  int res = 0;
  FORE(e, S) {
    // DEBUG(e)
    if (e.length() == 1)
      res += e[0] - '0';
    else
      res += mul(e);
  }
  return res;
}

int main(void) {
  scanf("%s", strbuf);
  S = strbuf;
  scanf("%d", &ans);
  int ltr = leftToRightCalc(S);
  int mf = mulFirstCalc(S);
  // DEBUG(ltr)DEBUG(mf)
  if (ans == mf && ans != ltr) {
    puts("M");
  } else if (ans != mf && ans == ltr) {
    puts("L");
  } else if (ans == mf && ans == ltr) {
    puts("U");
  } else {
    puts("I");
  }
}

Error: WA

Fixed submission:

#include <algorithm>
#include <bitset>
#include <cassert>
#include <climits>
#include <cmath>
#include <complex>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;
#define REP(i, n) for (int(i) = 0; (i) < (n); (i)++)
#define FOR(i, a, b) for (int(i) = (a); (i) < (b); (i)++)
#define RREP(i, a) for (int(i) = (a)-1; (i) >= 0; (i)--)
#define FORR(i, a, b) for (int(i) = (a)-1; (i) >= (b); (i)--)
#define DEBUG(C) cerr << #C << " = " << C << endl;
using LL = long long;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<LL>;
using VVL = vector<VL>;
using VD = vector<double>;
using VVD = vector<VD>;
using PII = pair<int, int>;
using PDD = pair<double, double>;
using PLL = pair<LL, LL>;
using VPII = vector<PII>;
template <typename T> using VT = vector<T>;
#define ALL(a) begin((a)), end((a))
#define RALL(a) rbegin((a)), rend((a))
#define SORT(a) sort(ALL((a)))
#define RSORT(a) sort(RALL((a)))
#define REVERSE(a) reverse(ALL((a)))
#define MP make_pair
#define FORE(a, b) for (auto &&a : (b))
#define FIND(s, e) ((s).find(e) != (s).end())
#define EB emplace_back
template <typename T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}

const int INF = 1e9;
const int MOD = INF + 7;
const LL LLINF = 1e18;

char strbuf[1000];
string s;
int ans;

int leftToRightCalc(string s) {
  const int L = s.length();
  int res = s[0] - '0';
  bool premul = false;
  FOR(i, 1, L) {
    if (isdigit(s[i])) {
      if (premul) {
        res *= s[i] - '0';
      } else {
        res += s[i] - '0';
      }
    } else {
      premul = s[i] == '*';
    }
  }
  return res;
}
int mulFirstCalc(string s) {
  string news = "";
  int res = 0;
  REP(i, s.length()) {
    if (isdigit(s[i]) || s[i] == '+') {
      news += s[i];
    } else {
      char c = news.back() - '0';
      news.pop_back();
      while (news.back() == '+')
        news.pop_back();
      char c2 = s[++i] - '0';
      res += (c * c2);
    }
  }
  // DEBUG(news)
  return res + leftToRightCalc(news);
}

int main(void) {
  scanf("%s", strbuf);
  s = strbuf;
  scanf("%d", &ans);
  int ltr = leftToRightCalc(s);
  int mf = mulFirstCalc(s);
  // DEBUG(ltr) DEBUG(mf)
  if (ans == mf && ans != ltr) {
    putchar('M');
  } else if (ans != mf && ans == ltr) {
    putchar('L');
  } else if (ans == mf && ans == ltr) {
    putchar('U');
  } else {
    putchar('I');
  }
}