思路:动态规划。参考http://www.cnblogs.com/grandyang/p/4299608.html
class Solution {
public:
int numTrees(int n) {
vector<int> dp(n+1,0);
dp[0]=1;
dp[1]=1;
for(int i=2;i<=n;++i)
for(int j=0;j<i;++j)
dp[i]+=dp[j]*dp[i-j-1];
return dp[n];
}
};