Problem p03986 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <cmath>
#include <complex>
#include <cstdio>
#include <deque>
#include <float.h>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <string.h>
#include <string>
#include <tuple>
#include <unordered_map>
#include <vector>

using namespace std;
#define int long long

#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
#define MEM_SIZE 10000
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()
// int DP[MEM_SIZE][MEM_SIZE] = {0};
// int GMEM[MEM_SIZE][MEM_SIZE] = {0};
template <typename T> void DEBUG(T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << e << std::endl;
}
template <typename T> void DEBUG(const std::vector<T> &v) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &e : v) {
    std::cout << e << " ";
  }
  std::cout << std::endl;
}
template <typename T> void DEBUG(const std::vector<std::vector<T>> &vv) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &v : vv) {
    DEBUG(v);
  }
}
template <class T, class X> void DEBUG(X d, T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << d << " ";
  DEBUG(e);
};
template <class T> void corner(bool flg, T hoge) {
  if (flg) {
    cout << hoge << endl;
    exit(0);
  }
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}
template <typename T> using matrix = vector<vector<T>>;

// FUNCTION

void solve(void) {
  string str;
  cin >> str;
  int D = 0;
  int cnt_s = 0, cnt_t = 0;
  int N = str.size();
  stack<char> st;
  st.push(str[0]);
  for (int i = 1; i < str.size(); i++) {
    if (str[i] == 'S') {
      st.push('S');
    } else {
      if (st.top() == 'S') {

        st.pop();
      } else {
        st.push('T');
      }
    }
  }

  cout << st.size() << endl;

  return;
}

int32_t main(int32_t argc, const char *argv[]) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(9);
  solve();

  return 0;
}


Fixed submission:
#include <algorithm>
#include <array>
#include <cmath>
#include <complex>
#include <cstdio>
#include <deque>
#include <float.h>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdio.h>
#include <string.h>
#include <string>
#include <tuple>
#include <unordered_map>
#include <vector>

using namespace std;
#define int long long

#define I32_MAX 2147483647
#define I64_MAX 9223372036854775807LL
#define I64_MAX2 1223372036854775807LL
#define INF I64_MAX2
#define MOD 1000000007
#define MEM_SIZE 10000
#define DEBUG_OUT true
#define ALL(x) (x).begin(), (x).end()
// int DP[MEM_SIZE][MEM_SIZE] = {0};
// int GMEM[MEM_SIZE][MEM_SIZE] = {0};
template <typename T> void DEBUG(T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << e << std::endl;
}
template <typename T> void DEBUG(const std::vector<T> &v) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &e : v) {
    std::cout << e << " ";
  }
  std::cout << std::endl;
}
template <typename T> void DEBUG(const std::vector<std::vector<T>> &vv) {
  if (DEBUG_OUT == false)
    return;
  for (const auto &v : vv) {
    DEBUG(v);
  }
}
template <class T, class X> void DEBUG(X d, T e) {
  if (DEBUG_OUT == false)
    return;
  std::cout << d << " ";
  DEBUG(e);
};
template <class T> void corner(bool flg, T hoge) {
  if (flg) {
    cout << hoge << endl;
    exit(0);
  }
}
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) {
  return a < b && (a = b, true);
}
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) {
  return a > b && (a = b, true);
}
template <typename T> using matrix = vector<vector<T>>;

// FUNCTION

void solve(void) {
  string str;
  cin >> str;
  int D = 0;
  int cnt_s = 0, cnt_t = 0;
  int N = str.size();
  stack<char> st;
  st.push(str[0]);
  for (int i = 1; i < str.size(); i++) {
    if (str[i] == 'S') {
      st.push('S');
    } else {
      if (st.empty()) {
        st.push('T');
      } else if (st.top() == 'S') {

        st.pop();
      } else {
        st.push('T');
      }
    }
  }

  cout << st.size() << endl;

  return;
}

int32_t main(int32_t argc, const char *argv[]) {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  std::cout << std::fixed;
  std::cout << std::setprecision(9);
  solve();

  return 0;
}