Unique Binary Search Trees II

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

Example :

Given A = 3, there are a total of 5 unique BST’s.


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

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