foo.cpp:1:1: error: 'include' does not name a type
 include"iostream"
 ^~~~~~~
foo.cpp:11:1: error: 'string' does not name a type
 string f(string a,int b,int e) //截取子串 STL 里面貌似有个函数但是记不到了。。。
 ^~~~~~
foo.cpp:19:13: error: 'string' was not declared in this scope
 node* build(string s1,string s2)
             ^~~~~~
foo.cpp:19:23: error: 'string' was not declared in this scope
 node* build(string s1,string s2)
                       ^~~~~~
foo.cpp:19:32: error: expression list treated as compound expression in initializer [-fpermissive]
 node* build(string s1,string s2)
                                ^