add unique_bsts problem

Author: sagivo

README.md

@@ -13,6 +13,7 @@ This is my way of saying - change your interview style. There are lots of smart
 
 | problem | solution |
 |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| [Unique binary search trees](https://leetcode.com/problems/unique-binary-search-trees/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/house-robber.rb) |
 | [House Robber](https://leetcode.com/problems/house-robber/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/house-robber.rb) |
 | [Array Product](https://leetcode.com/problems/product-of-array-except-self/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/product-of-array.rb) |
 | [Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/top-k-elements.rb) |

src/unique_bsts.rb

@@ -0,0 +1,14 @@
+def unique_bsts(n)
+ a = Array.new(n+1, 0)
+ a[0] = a[1] = 1
+
+ (2..n).each do |i|
+ (1..i).each do |j|
+ a[i] += (a[j-1] * a[i-j])
+ end
+ end
+
+ return a[n]
+end
+
+p unique_bsts(4) # => 14