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  1. Prepare
  2. Functional Programming
  3. Memoization and DP
  4. Number of Binary Search Tree
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Number of Binary Search Tree

  • h503689341
     + 1 comment

    I am just a fresh man in Haskell, and I have already know the lazy evaluation, so I can't understand why my following answer can't get result for the input of 100. Any advice will be appreciated! Thx a lot!

    nLevelBST n = rem (sum([nthLevelBST 1 t n | t <- [1..n]])) (10^8 + 7)

nthLevelBST l m r
    | l == m && m == r = 1
    | l == m && m /= r = sum([nthLevelBST (m+1) mr r | mr <- [(m+1)..r]])
    | l /= m && m == r = sum([nthLevelBST l ml (m-1) | ml <- [l..(m-1)]])
    | otherwise = (sum([nthLevelBST l ml (m-1) | ml <- [l..(m-1)]])) * (sum([nthLevelBST (m+1) mr r | mr <- [(m+1)..r]]))