Problem p00087 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
typedef tuple<int, int> duo;

#define TT template <typename T>
#define ET(T) typename T::value_type
TT inline T sq(T x) { return x * x; }
TT inline T In() {
  T x;
  cin >> x;
  return x;
}
TT inline int ubnd(T &v, ET(T) x) {
  return upper_bound(begin(v), end(v), x) - begin(v);
}
TT inline int lbnd(T &v, ET(T) x) {
  return lower_bound(begin(v), end(v), x) - begin(v);
}
TT inline void sort(T &v) { sort(begin(v), end(v)); }
TT inline void revs(T &v) { reverse(begin(v), end(v)); }
TT inline void uniq(T &v) {
  sort(v);
  v.erase(unique(begin(v), end(v)), end(v));
}
TT inline void inpt(T &v, int n, function<ET(T)()> f = In<ET(T)>) {
  for (v.reserve(n); n--; v.emplace_back(f()))
    ;
}
TT inline void show(T &v, string d = " ", string e = "\n") {
  int i = 0;
  for (auto &x : v)
    i++ && (cout << d), cout << x;
  cout << e;
}
#undef TT
#undef ET

inline void fast_io() {
  ios::sync_with_stdio(0);
  cin.tie(0);
}
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
inline string instr() {
  string x;
  cin >> x;
  return x;
}
inline ll pow_mod(ll a, ll k, ll m) {
  ll r = 1;
  for (; k > 0; a = a * a % m, k >>= 1)
    if (k & 1)
      r = r * a % m;
  return r;
}
inline ll mod_inv(ll a, ll p) { return pow_mod(a, p - 2, p); }

const int dx[] = {0, 0, 1, -1, 1, 1, -1, -1};
const int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
const int Mod = 1000000000;

bool is_digit(const string &s) {
  if (s.size() == 1)
    return isdigit(s[0]);
  return (s[0] == '+' || s[0] == '-') && isdigit(s[1]);
}

int main() {
  string line;
  while (getline(cin, line)) {
    stringstream ss(line);
    string e;
    stack<ld> st;
    while (ss >> e) {
      if (is_digit(e))
        st.push(stof(e));
      else {
        ld a, b;
        a = st.top();
        st.pop();
        b = st.top();
        st.pop();
        if (e == "+")
          st.push(b + a);
        if (e == "-")
          st.push(b - a);
        if (e == "*")
          st.push(b * a);
        if (e == "/")
          st.push(b / a);
      }
    }
    printf("%Lf\n", st.top());
  }
  return (0);
}

// priority_queue queue deque front top push pop map set stringstream
// max_element min_element insert count find get tie make_tuple

Fixed submission:
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;
typedef tuple<int, int> duo;

#define TT template <typename T>
#define ET(T) typename T::value_type
TT inline T sq(T x) { return x * x; }
TT inline T In() {
  T x;
  cin >> x;
  return x;
}
TT inline int ubnd(T &v, ET(T) x) {
  return upper_bound(begin(v), end(v), x) - begin(v);
}
TT inline int lbnd(T &v, ET(T) x) {
  return lower_bound(begin(v), end(v), x) - begin(v);
}
TT inline void sort(T &v) { sort(begin(v), end(v)); }
TT inline void revs(T &v) { reverse(begin(v), end(v)); }
TT inline void uniq(T &v) {
  sort(v);
  v.erase(unique(begin(v), end(v)), end(v));
}
TT inline void inpt(T &v, int n, function<ET(T)()> f = In<ET(T)>) {
  for (v.reserve(n); n--; v.emplace_back(f()))
    ;
}
TT inline void show(T &v, string d = " ", string e = "\n") {
  int i = 0;
  for (auto &x : v)
    i++ && (cout << d), cout << x;
  cout << e;
}
#undef TT
#undef ET

inline void fast_io() {
  ios::sync_with_stdio(0);
  cin.tie(0);
}
inline int in() {
  int x;
  scanf("%d", &x);
  return x;
}
inline string instr() {
  string x;
  cin >> x;
  return x;
}
inline ll pow_mod(ll a, ll k, ll m) {
  ll r = 1;
  for (; k > 0; a = a * a % m, k >>= 1)
    if (k & 1)
      r = r * a % m;
  return r;
}
inline ll mod_inv(ll a, ll p) { return pow_mod(a, p - 2, p); }

const int dx[] = {0, 0, 1, -1, 1, 1, -1, -1};
const int dy[] = {1, -1, 0, 0, 1, -1, 1, -1};
const int Mod = 1000000000;

bool is_digit(const string &s) {
  if (isdigit(s[0]))
    return true;
  return (s[0] == '+' || s[0] == '-') && isdigit(s[1]);
}

int main() {
  string line;
  while (getline(cin, line)) {
    stringstream ss(line);
    string e;
    stack<ld> st;
    while (ss >> e) {
      if (is_digit(e))
        st.push(stof(e));
      else {
        ld a, b;
        a = st.top();
        st.pop();
        b = st.top();
        st.pop();
        if (e == "+")
          st.push(b + a);
        if (e == "-")
          st.push(b - a);
        if (e == "*")
          st.push(b * a);
        if (e == "/")
          st.push(b / a);
      }
    }
    printf("%Lf\n", st.top());
  }
  return (0);
}

// priority_queue queue deque front top push pop map set stringstream
// max_element min_element insert count find get tie make_tuple