Compilare: user.cpp: In function ‘void pushToTrie(TrieNode*, int, std::string&)’: user.cpp:41:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (letter == str.length()) { // finished the current word ^ user.cpp: In function ‘bool popFromTrie(TrieNode*, TrieNode*, int, std::string&)’: user.cpp:55:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (letter == str.length()) { // finished the current word ^ user.cpp: In function ‘int getNumberOfWords(TrieNode*, int, std::string&)’: user.cpp:81:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (letter == str.length()) { // we have reached the end of the word ^ user.cpp: In function ‘int getLengthOfPrefix(TrieNode*, int, std::string&)’: user.cpp:97:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (letter == str.length() || root->child.find(str[letter]) == root->child.end()) { // if I've finished the current word or ^