unique-binary-search-trees

图示

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];
    }
};