我用的栈和字符串，为啥会RE

#include <bits/stdc++.h>
using namespace std;
int main()
{
	stack <char> a;
	string b;
	cin>>b;
	if(b.at(1)=='-')
	{
		cout<<"-";
	}
	for(int i=1;i<=b.size();i++)
	{
		if(b.at(1)=='-')
		{
			continue;
		}else{
//			char c;
			a.push(b.at(i));
		}
	}
	for(int i=1;i<=a.size();i++)
	{
		if(a.top()=='0')
		{
			continue;
		}else{
			cout<<a.top();
			a.pop();
		}
	}
	return 0;
}