foo.cc: In constructor 'BinTree::BinTree(const char*, const char*)':
foo.cc:20:9: error: 'strlen' was not declared in this scope
   20 |   int n=strlen(pre);
      |         ^~~~~~
foo.cc:2:1: note: 'strlen' is defined in header '<cstring>'; did you forget to '#include <cstring>'?
    1 | #include<iostream>
  +++ |+#include <cstring>
    2 | using namespace std;

[Hydro](https://hydro.ac)提供评测服务