View Compilation Error

0_0_21852320_24385.cpp: In function 'int main()':
0_0_21852320_24385.cpp:53:59: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'
         for(int i = 1; i <= n; i ++){if(!dfn[i])tarjan(i,1);}
                                                           ^
0_0_21852320_24385.cpp:22:6: note: in passing argument 2 of 'void tarjan(int, int&)'
 void tarjan(int u,int &len)
      ^