输入：

10
vca
ab*
bz*
c**
d*f
z*u
f*k
k*h
h**
ud*

理应输出：

vcabzudfkh

实际报错：

terminate called after throwing an instance of 'std::out_of_range'
  what():  basic_string::erase: __pos (which is 18446744073709551615) > this->size() (which is 9)```