Unique Binary Search Trees .3.cpp

class Solution {
public:
    int numTrees(int n) {
        vector<int>f(n+1,0);
        f[0]=1;
        for(int i=1;i<=n;i++){
            for(int j=0;j<=i-1;j++)
                f[i]+=f[j]*f[i-1-j];
        }
        return f[n];
    }
};