error: invalid conversion from 'char' to 'const char*'
A: You are trying to use a char in place of a string.
You can convert a char into a string if necessary by doing something like this:
char c = 'Q';
string s;
s += c; // "Q"
or,
string s = string("") + c;
Q: How do I "mark" a letter as being used?
A: We don't want to say exactly how to do it; the details are up to you.
Come up with a way to store this information in a collection, or in some existing collection in the code.
Q: How efficient must my computer word search be?
Mine takes up to a few minutes to run; is this okay?
A: It should finish running almost immediately.
If your solution takes more than a few seconds to run, something is wrong and you should fix it if you want to receive full credit.