[Leetcode] Unique Binary Search Trees @ Python

Given N, how many structurally unique BST's (binary search trees) that store values 1...N?

For example,

Given n = 3, there is a total of 5 unique BST ' s.

   1         3     3      2      1    \//      /\           3     2     1      1   3      2    /     /       \                    2     1         2                 3

The main idea is simple, is to calculate how many different two forks looking for a tree to meet the pre-sequence traversal is 1,2,3...N conditions

Idea: Each tree is composed of root node, left subtree, right subtree, once the root node is determined to be X,

Zuozi can only be constituted by 1,2,3...x-1, and the right subtree is composed of X+1,X+2...N, which can
[Method 1] fall generation solution;

classSolution:#@return An integer    defnumtrees (self, n): DP= [1, 1, 2]        ifN < 3:returnDp[n] DP+ = [0 forIinchRange (n-2)]         forIinchRange (3, n+1):             forJinchRange (i): Dp[i]+ = Dp[j]*dp[i-j-1]        returnDp[n]

[Method 2] Recursive solution

class Solution:     # @return an integer    def numtrees (self, N):         = [1, 1, 2]        ifreturn  dp[n]        = 0          for inch range (N):               + = Self.numtrees (i) *self.numtrees (n-i-1          )return ans

[Leetcode] Unique Binary Search Trees @ Python