DP

$ 时间复杂度O(N^2),空间复杂度O(N)$

参考文献

lc究极班

AC代码

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