mexc.cpp: In constructor 'trie::trie()': mexc.cpp:16: error: 'nullptr' was not declared in this scope mexc.cpp: In function 'void add(trie*, int)': mexc.cpp:24: warning: comparison between signed and unsigned integer expressions mexc.cpp:26: error: 'nullptr' was not declared in this scope mexc.cpp: In function 'void del(trie*, int)': mexc.cpp:49: warning: comparison between signed and unsigned integer expressions mexc.cpp:51: error: 'nullptr' was not declared in this scope mexc.cpp: In function 'int main()': mexc.cpp:67: warning: comparison between signed and unsigned integer expressions mexc.cpp:79: warning: comparison between signed and unsigned integer expressions