Problem p04030 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <bits/stdc++.h>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

/**** Type Define ****/

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P> Q;

/**** Const List   ****/

const ll INF = 1LL << 60;
const ll mod = 1000000007;
const ll dx[4] = {1, 0, -1, 0};
const ll dy[4] = {0, -1, 0, 1};

/**** General Functions ****/

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll extgcd(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1, y = 0;
    return a;
  }
  ll q = a / b, g = extgcd(b, a - q * b, x, y);
  ll z = x - q * y;
  x = y;
  y = z;
  return g;
}

ll invmod(ll a, ll m) { // a^-1 mod m
  ll x, y;
  extgcd(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

ll nCk(ll n, ll k, ll mod) {
  ll ans = 1;
  for (ll i = n, j = 1; j <= k; i--, j++)
    ans = (((ans * i) % mod) * invmod(j, mod)) % mod;
  return ans;
}

ll lmin(ll a, ll b) { return a > b ? b : a; };

ll lmax(ll a, ll b) { return a > b ? a : b; };

ll lsum(ll a, ll b) { return a + b; };

/**** Zip ****/

template <typename T> class Zip {
  vector<T> d;
  bool flag;

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) { // T need to have operator < !!
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return (ll)d.size();
  }
};

/**** Union Find ****/

class UnionFind {
  vector<ll> par, rank; // par > 0: number, par < 0: -par
public:
  void init(ll n) {
    par.resize(n, 1);
    rank.resize(n, 0);
  }
  ll getSize(ll x) { return par[find(x)]; }
  ll find(ll x) {
    if (par[x] > 0)
      return x;
    return -(par[x] = -find(-par[x]));
  }
  void merge(ll x, ll y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[y] += par[x];
      par[x] = -y;
    } else {
      par[x] += par[y];
      par[y] = -x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }
  bool isSame(ll x, ll y) { return find(x) == find(y); }
};

/**** Segment Tree ****/

template <typename T> class SegmentTree {
  ll n;
  vector<T> node;
  function<T(T, T)> fun, fun2;
  bool customChange;
  T outValue, initValue;

public:
  void init(ll num, function<T(T, T)> resultFunction, T init, T out,
            function<T(T, T)> changeFunction = NULL) {
    // changeFunction: (input, beforevalue) => newvalue
    fun = resultFunction;
    fun2 = changeFunction;
    customChange = changeFunction != NULL;
    n = 1;
    while (n < num)
      n *= 2;
    node.resize(2 * n - 1, init);
    outValue = out;
    initValue = init;
  }
  void valueChange(ll num, T value) {
    num += n - 1;
    if (customChange)
      node[num] = fun2(value, node[num]);
    else
      node[num] = value;
    while (num > 0)
      num = (num - 1) / 2,
      node[num] = fun(node[num * 2 + 1], node[num * 2 + 2]);
  }
  T rangeQuery(ll a, ll b, ll l = 0, ll r = -1, ll k = 0) { // [a, b)
    if (r == -1)
      r = n;
    if (a <= l && r <= b)
      return node[k];
    if (b <= l || r <= a)
      return outValue;
    ll mid = (l + r) / 2;
    return fun(rangeQuery(a, b, l, mid, 2 * k + 1),
               rangeQuery(a, b, mid, r, 2 * k + 2));
  }
};

/**** LIS ****/
ll lis(ll *a, ll n, ll *dp) {
  fill(dp, dp + n, INF); // INFを代入
  for (ll i = 0; i < n; i++)
    *lower_bound(dp, dp + n, a[i]) = a[i];
  return (ll)(lower_bound(dp, dp + n, INF) - dp);
}

/**** main function ****/
string s;
string ans;
int main() {
  cin >> s;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') {
      ans.push_back('0');
    } else if (s[i] == '1') {
      ans += ('1');
    } else {
      if (s.size() != 0)
        ans.pop_back();
    }
  }
  cout << ans << endl;
}


Fixed submission:
#include <algorithm>
#include <bits/stdc++.h>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <stack>
#include <string>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;

/**** Type Define ****/

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<ll, P> Q;

/**** Const List   ****/

const ll INF = 1LL << 60;
const ll mod = 1000000007;
const ll dx[4] = {1, 0, -1, 0};
const ll dy[4] = {0, -1, 0, 1};

/**** General Functions ****/

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll extgcd(ll a, ll b, ll &x, ll &y) {
  if (b == 0) {
    x = 1, y = 0;
    return a;
  }
  ll q = a / b, g = extgcd(b, a - q * b, x, y);
  ll z = x - q * y;
  x = y;
  y = z;
  return g;
}

ll invmod(ll a, ll m) { // a^-1 mod m
  ll x, y;
  extgcd(a, m, x, y);
  x %= m;
  if (x < 0)
    x += m;
  return x;
}

ll nCk(ll n, ll k, ll mod) {
  ll ans = 1;
  for (ll i = n, j = 1; j <= k; i--, j++)
    ans = (((ans * i) % mod) * invmod(j, mod)) % mod;
  return ans;
}

ll lmin(ll a, ll b) { return a > b ? b : a; };

ll lmax(ll a, ll b) { return a > b ? a : b; };

ll lsum(ll a, ll b) { return a + b; };

/**** Zip ****/

template <typename T> class Zip {
  vector<T> d;
  bool flag;

public:
  Zip() { flag = false; }
  void add(T x) {
    d.push_back(x);
    flag = true;
  }
  ll getNum(T x) { // T need to have operator < !!
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return lower_bound(d.begin(), d.end(), x) - d.begin();
  }
  ll size() {
    if (flag) {
      sort(d.begin(), d.end());
      d.erase(unique(d.begin(), d.end()), d.end());
      flag = false;
    }
    return (ll)d.size();
  }
};

/**** Union Find ****/

class UnionFind {
  vector<ll> par, rank; // par > 0: number, par < 0: -par
public:
  void init(ll n) {
    par.resize(n, 1);
    rank.resize(n, 0);
  }
  ll getSize(ll x) { return par[find(x)]; }
  ll find(ll x) {
    if (par[x] > 0)
      return x;
    return -(par[x] = -find(-par[x]));
  }
  void merge(ll x, ll y) {
    x = find(x);
    y = find(y);
    if (x == y)
      return;
    if (rank[x] < rank[y]) {
      par[y] += par[x];
      par[x] = -y;
    } else {
      par[x] += par[y];
      par[y] = -x;
      if (rank[x] == rank[y])
        rank[x]++;
    }
  }
  bool isSame(ll x, ll y) { return find(x) == find(y); }
};

/**** Segment Tree ****/

template <typename T> class SegmentTree {
  ll n;
  vector<T> node;
  function<T(T, T)> fun, fun2;
  bool customChange;
  T outValue, initValue;

public:
  void init(ll num, function<T(T, T)> resultFunction, T init, T out,
            function<T(T, T)> changeFunction = NULL) {
    // changeFunction: (input, beforevalue) => newvalue
    fun = resultFunction;
    fun2 = changeFunction;
    customChange = changeFunction != NULL;
    n = 1;
    while (n < num)
      n *= 2;
    node.resize(2 * n - 1, init);
    outValue = out;
    initValue = init;
  }
  void valueChange(ll num, T value) {
    num += n - 1;
    if (customChange)
      node[num] = fun2(value, node[num]);
    else
      node[num] = value;
    while (num > 0)
      num = (num - 1) / 2,
      node[num] = fun(node[num * 2 + 1], node[num * 2 + 2]);
  }
  T rangeQuery(ll a, ll b, ll l = 0, ll r = -1, ll k = 0) { // [a, b)
    if (r == -1)
      r = n;
    if (a <= l && r <= b)
      return node[k];
    if (b <= l || r <= a)
      return outValue;
    ll mid = (l + r) / 2;
    return fun(rangeQuery(a, b, l, mid, 2 * k + 1),
               rangeQuery(a, b, mid, r, 2 * k + 2));
  }
};

/**** LIS ****/
ll lis(ll *a, ll n, ll *dp) {
  fill(dp, dp + n, INF); // INFを代入
  for (ll i = 0; i < n; i++)
    *lower_bound(dp, dp + n, a[i]) = a[i];
  return (ll)(lower_bound(dp, dp + n, INF) - dp);
}

/**** main function ****/
string s;
string ans;
int main() {
  cin >> s;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') {
      ans.push_back('0');
    } else if (s[i] == '1') {
      ans += ('1');
    } else {
      if (ans.size() != 0)
        ans.pop_back();
    }
  }
  cout << ans << endl;
}