foo.cpp: In function 'void preBFS()':
foo.cpp:34:30: error: 'memset' was not declared in this scope
     memset(vis, 0, sizeof vis);
                              ^
foo.cpp:39:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i = 0; i < H[a].size(); i++)
                                      ^
foo.cpp: In function 'bool walkable(const int&)':
foo.cpp:50:36: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0; i < G[src].size(); i++)
                                    ^
foo.cpp: In function 'void SPFA()':
foo.cpp:58:30: error: 'memset' was not declared in this scope
     memset(vis, 0, sizeof vis);
                              ^
foo.cpp:64:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i = 0; i < G[a].size(); i++)    
                                      ^