leetcode96

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 j=1;j<=i;j++){
                f[i]+=f[j-1]*f[i-j];
            }
        }
        return f[n];
    }
};