Segmentation Fault at runtime in c++ -

I have determined that the error occurs when I toString () my Linkedist class , more specifically when the line 8 or 9 is executed

  string LinkList :: toString () const {int traverse = 1; Node * key = head; Wire south; While (size> = trousers) {sout + = "value =" + ---> "+"; That = that- & gt; next; Cross ++; } Return south; }   
 This linkedlist uses a node class to store data that looks like this, and Linkedist class has its own integer shape variable and an indicator for the first node < / P> 
  class node {friend class linked list; Private: std :: string s; Node * next; Node (std :: string street, node * nextn); };   
 I'm guessing that this is some dirty mistake in my part, but I can not understand it by stalking on it or by reading the tutorial again on the signs.   
 
 The code is defective below. Please add a log for the blank check  
  that = that-> Next (if he == blank) {// std :: cout & lt; & Lt; "A null node was found \ n"; break; }