CompileError!

main.cpp: In function \xe2\x80\x98TreeNode* buildTree(const std::map<int, std::pair<int, char> >&)\xe2\x80\x99:\nmain.cpp:40:5: error: \xe2\x80\x98set\xe2\x80\x99 was not declared in this scope\n   40 |     set<int> childNodes;\n      |     ^~~\nmain.cpp:5:1: note: \xe2\x80\x98std::set\xe2\x80\x99 is defined in header \xe2\x80\x98<set>\xe2\x80\x99; did you forget to \xe2\x80\x98#include <set>\xe2\x80\x99?\n    4 | #include <map>\n  +++ |+#include <set>\n    5 | \nmain.cpp:40:9: error: expected primary-expression before \xe2\x80\x98int\xe2\x80\x99\n   40 |     set<int> childNodes;\n      |         ^~~\nmain.cpp:42:9: error: \xe2\x80\x98childNodes\xe2\x80\x99 was not declared in this scope\n   42 |         childNodes.insert(it->second.first);\n      |         ^~~~~~~~~~\nmain.cpp:46:13: error: \xe2\x80\x98childNodes\xe2\x80\x99 was not declared in this scope\n   46 |         if (childNodes.find(it->first) == childNodes.end()) {\n      |             ^~~~~~~~~~\n