foo.cc: In constructor 'BinTree::BinTree(const char*, const char*)':
foo.cc:21:11: error: 'strlen' was not declared in this scope
   21 |   int n = strlen(pre);
      |           ^~~~~~
foo.cc:3:1: note: 'strlen' is defined in header '<cstring>'; did you forget to '#include <cstring>'?
    2 | #include<queue>
  +++ |+#include <cstring>
    3 | using namespace std;

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