431C — k-Tree:I have not yet exposed to dynamic programming, so I solved it recursively and got TLE in test case #7. I'm having the hard time(nearly impossible) to trace the bug. Could anybody take a look at the code ? I doubt whether TLE is caused by my poor recursive method or my misunderstanding about the logic of mod 10^9 + 7. If bug is the later, how can I fix it?