catalan number
不过不懂怎么推导TAT
用递推来实现吧
class Solution { public: int numTrees(int n) { vector<int> f(n+1 , 0); f[0] = 1; f[1] = 1; for(int i = 2 ; i <= n ; ++i) { for(int k = 1 ; k <= i ; ++k) { f[i] += f[k-1] * f[i-k]; } } return f[n]; } };