View Compilation Error

0_0_31758584_27110.cpp:19:11: error: array bound is not an integer constant before ']' token
 node e[2*n];
           ^
0_0_31758584_27110.cpp: In function 'void add(int, int)':
0_0_31758584_27110.cpp:21:2: error: 'e' was not declared in this scope
  e[idx].u=a;
  ^
0_0_31758584_27110.cpp: In function 'void dfs(int)':
0_0_31758584_27110.cpp:32:21: error: 'e' was not declared in this scope
  for(i=h[u];i!=-1;i=e[i].to)
                     ^
0_0_31758584_27110.cpp: In function 'int dfs1(int, int, int)':
0_0_31758584_27110.cpp:41:21: error: 'e' was not declared in this scope
  for(i=h[u];i!=-1;i=e[i].to){
                     ^
0_0_31758584_27110.cpp: In function 'int dfs2(int, int)':
0_0_31758584_27110.cpp:62:21: error: 'e' was not declared in this scope
  for(i=h[u];i!=-1;i=e[i].to){
                     ^