public:

int numTrees(int n) {

if (n==0) return 1;

if (n==1) return 1;

vector<int> nTree;

nTree.push_back(numTrees(0));

nTree.push_back(numTrees(1));

for(int i=2;i<n+1;i++){

int tmp;

tmp=0;

for(int j=0;j<i;j++){

tmp+=nTree[j]*nTree[i-1-j];

}

nTree.push_back(tmp);

}

return nTree[n];

}