我的代码c++报错了，好像是运行错误，很奇怪，这是为啥

#include<iostream>
#include<string>
using namespace std;
struct tre{
	int l,r;
	char d;
}a[50];
int top=0;
void t(string x,string y,char z,int b)
{
	a[++top].d=x[0];
	if(z=='l') a[b].l=top;
	if(z=='r') a[b].r=top;
	int c=y.find(x[0]);
	t(x.substr(1,c),y.substr(0,c-1),'l',top);
	t(x.substr(c+1,x.size()-c),y.substr(c+1,y.size()-c),'r',top);
}
void out(int b)
{
	if(!b) return;
	out(a[b].l);
	out(a[b].r);
	cout<<a[b].d;
}
int main()
{
	string _1,_2;
	cin>>_1>>_2;
	t(_1,_2,'n',0);
	out(1);
	return 0;
}

报错信息：

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::substr: __pos (which is 1) > this->size() (which is 0)