s.c_str(), key[i].c_str()

These are two string functions that return a null-terminated C-style character array (also known as a C-string) representing the contents of a C++ string object.

s.c_str(): This function returns a pointer to the underlying character array used by the string object 's'. The returned C-string is null-terminated, which means it ends with a null character '\0'. This function is useful when we need to pass the contents of a C++ string object to a C-style string function that expects a null-terminated string as input.

key[i].c_str(): This function returns a pointer to the C-string representation of the i-th element of the string array 'key'. This is similar to s.c_str(), but instead of returning the entire string, it returns the C-string representation of a single element of the string array. This function is useful when we need to pass a single string element of a C++ string array to a C-style string function.