2020.7.15 刷

class Solution {
    public int numTrees(int n) {
        int[] res = new int[n + 1];
        if(n == 1)return 1;
        if(n == 2)return 2;
        res[0] = 1;
        res[1] = 1;
        res[2] = 2;
        for (int i = 3; i <= n; i++) {
            for (int j = 1; j <= i / 2; j++) {
                res[i] += res[i - j] * res[j - 1];
            }
            res[i] *= 2;
            if(i % 2 == 1)
                res[i] += res[(i - 1) / 2] * res[(i - 1) / 2];
        }
        return res[n];
    }
}

class Solution {
    public int numTrees(int n) {
        int[] dp = new int[n+1];
        dp[0] = 1;
        dp[1] = 1;
        
        for(int i = 2; i < n + 1; i++)
            for(int j = 1; j < i + 1; j++) 
                dp[i] += dp[j-1] * dp[i-j];
        
        return dp[n];
    }
}

作者：guanpengchn
链接：https://leetcode-cn.com/problems/unique-binary-search-trees/solution/hua-jie-suan-fa-96-bu-tong-de-er-cha-sou-suo-shu-b/
来源：力扣（LeetCode）
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

class Solution:
    def numTrees(self, n: int) -> int:
        dp = [1, 1, 2]
        for i in range (3, n + 1):
            res = 0
            for j in range (1, (int)(i/2 + 1)):
                res += dp[j - 1] * dp[i - j]
            res *= 2
            if i % 2 == 1:
                res += dp[(int)((i-1)/ 2)] ** 2
            dp.append(res)
        return dp[n]