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

For example, Given n = 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
	 *