add support for negative integer

Author: keineahnung2345

string_is_number.cpp

@@ -5,12 +5,18 @@ using namespace std;
 
 //https://stackoverflow.com/questions/4654636/how-to-determine-if-a-string-is-a-number-with-c
 
+//support positive and negative integer
 bool is_number(const std::string& s){
-    return !s.empty() && s.find_first_not_of("0123456789") == std::string::npos;
+    if(s.empty()) return false;
+    if(s.size() > 1 && s[0] == '-' && s.find_first_not_of("0123456789", 1) == std::string::npos) return true;
+    return s.find_first_not_of("0123456789") == std::string::npos;
 };
 
 int main() {
     cout << is_number("123") << endl;    //1
+    cout << is_number("-123") << endl;    //1
+    cout << is_number("-123abc") << endl;    //0
+    cout << is_number("-") << endl;    //0
     cout << is_number("abc") << endl;    //0
     cout << is_number("123abc") << endl; //0
     return 0;