Invalid file path when problem title contains special character

[hucancode]

There is a problem with special character in their title will result in invalid path For example, this problem: Is Graph Bipartite? Will result in this path leetcode/problems/is_graph_bipartite?/solution.rs which contains invalid character ? (at least it is invalid in Windows)

[joshcai]

Thanks for reporting @hucancode! Apologies for the delay on getting to this, but this should be fixed now.