View Compilation Error

In file included from GCC4.9.2/x86_64-w64-mingw32/include/c++/cassert:43:0,
                 from 0_0_20200777_8787.cpp:7:
0_0_20200777_8787.cpp: In function 'void dfs(int, int)':
0_0_20200777_8787.cpp:44:9: error: 'key' was not declared in this scope
  assert(key[x].size() <= 1000);
         ^