/in/foo.cc: In function 'int dfs(int, int)':
/in/foo.cc:11:61: error: 'max' was not declared in this scope
             ret=max(ret, dfs(lson[u], i)+dfs(rson[u], x-i-1));
                                                             ^
/in/foo.cc:13:33: error: 'max' was not declared in this scope
     ret=max(ret, dfs(rson[u], x));     //不选u
                                 ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:19:28: error: 'memset' was not declared in this scope
     memset(f, -1, sizeof(f));
                            ^
/in/foo.cc:22:15: error: 'cin' was not declared in this scope
     int n, m; cin>>n>>m;
               ^~~
/in/foo.cc:29:5: error: 'cout' was not declared in this scope
     cout<<dfs(lson[0], m)<<endl;        //lson[0]是根
     ^~~~
/in/foo.cc:29:28: error: 'endl' was not declared in this scope
     cout<<dfs(lson[0], m)<<endl;        //lson[0]是根
                            ^~~~