Language: C++

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

#define Abs(x)((x) < 0 ? (x) * -1 : (x))

#define rep(x, y) for ((x) = 0; (x) < (y); (x)++)
#define repin(x, y) for ((x) = 0; (x) <= (y); (x)++)
#define nep(x, y) for ((x) = (y) - 1; 0 <= (x); (x)--)
#define nepi(x, y, z) for ((x) = (y) - 1; (z) <= (x); (x)--)
#define repi(x, y, z) for ((x) = (z); (x) < (y); (x)++)
#define repiin(x, y, z) for ((x) = (z); (x) <= (y); (x)++)
#define reps(x, y, z) for ((x) = 0; (x) < (y); (x) += (z))
#define repis(x, y, z, s) for ((x) = (z); (x) < (y); (x) += (s))
#define repiins(x, y, z, s) for ((x) = (z); (x) <= (y); (x) += (s))
#define repit(x) for (__typeof((x).begin()) it = (x).begin(); it != (x).end(); it++)
#define nepit(x) for (__typeof((x).rbegin()) it = (x).rbegin(); it != (x).rend(); it++)

#define All(x) (x).begin(),(x).end()

#define Mem0(x) memset(x, 0, sizeof(x))
#define Mem1(x) memset(x, -1, sizeof(x))

#define Unique(v) v.resize(unique(All(v)) - v.begin())

#define peq(p0, p1)(p0.first == p1.first && p0.second == p1.second)

#define End '\n'
#define Out(x) cout<<(x)<<End
#define OutP(p, sep, end) cout<<p.first<<sep<<p.second<<end

template<typename T> inline bool Max(T &x, const T &y) { return x < y ? x = y, 1 : 0; }
template<typename T> inline bool Min(T &x, const T &y) { return x > y ? x = y, 1 : 0; }
template<typename T> using V = vector<T>;
template<typename T> using VV = V<V<T> >;

#define pb push_back
#define mp make_pair
#define X first
#define Y second
#define lb std::lower_bound
#define ub std::upper_bound

#define lbi(v, x) lb(All(v), (x))-v.begin()
#define ubi(v, x) ub(All(v), (x))-v.begin()

static const ll MOD = 1e9 + 7;
static const double PI = 3.141592653589793;

#define LOCAL 0

int main()
{
#if LOCAL
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    freopen("debug.txt", "w", stderr);
#endif

    cin.tie(0);
    ios::sync_with_stdio(false);
	//std::cout.precision(18);

    string s;

	cin >> s;

	stack<char> st;
	string ans = "";

	int i, len = s.size();

	rep(i, len) {
		if (s[i] == '0') s.push('0');
		else if (s[i] == '1') s.push('1');
		else s.pop();
	}

	while (!s.empty()) {
		ans += s.top();

		s.pop();
	}

	std::reverse(All(ans));

	Out(ans);
	
	return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:77:36: error: 'std::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'push'
   77 |                 if (s[i] == '0') s.push('0');
      |                                    ^~~~
a.cc:78:41: error: 'std::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'push'
   78 |                 else if (s[i] == '1') s.push('1');
      |                                         ^~~~
a.cc:79:24: error: 'std::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'pop'
   79 |                 else s.pop();
      |                        ^~~
a.cc:83:26: error: 'std::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'top'
   83 |                 ans += s.top();
      |                          ^~~
a.cc:85:19: error: 'std::string' {aka 'class std::__cxx11::basic_string<char>'} has no member named 'pop'
   85 |                 s.pop();
      |                   ^~~