AC记录 https://hydro.ac/d/atcoder/record/66bea7623f788cb34ad04898

#include <bits/stdc++.h>
#include <algorithm>

#define endl '\n';


const int N = 1e6 + 10;
const int M = 2e6 + 10;
const int P = 998244353; 
const int Base = 3221225477;
const int INF = 0x3f3f3f3f3f3f3f3f; 
int mod(int x)
{
   return x % P;
}
using namespace std;
char a[N];
string s;
signed main()
{
	cin >> s;
	int pos = 0;
	for(int i = 0;i < s.size();i++)
	{
		if(s[i] == '1')
		{
			a[pos] = '1';
			pos++;
		}else if(s[i] == '0')
		{
			a[pos] = '0';
			pos++;
		}else if(s[i] == 'B')
		{
			if(pos > 0)
			{
				a[pos] = ' ';
				pos--;
			}else if(pos <= 0)
			{
				pos = 0;
			}
		}
	}
	for(int i = 0;i < pos;i++)
	{
		cout << a[i];
	}
	return 0;
}