Unique Binary Search Trees II


#1

Given an integer n, generate all structurally unique BST's (binary search trees) that store values 1 ... n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

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

#2

The below algorithm uses recursion to generate all the possible left sub trees and right sub trees at every i from 1 to n.

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func generateTrees(n int) []*TreeNode {
    if n==0{
        return []*TreeNode{}
    }
    return genTreeList(1, n)
}

func genTreeList(start, end int) []*TreeNode{
    fmt.Printf("start %d , end %d \n", start, end)
    list := make([]*TreeNode, 0)
    if start>end {
        list = append(list, nil)
    }
    
    for i:=start;i<=end;i++{
        leftList := genTreeList(start, i-1)
        rightList := genTreeList(i+1, end)
        for _, left := range leftList{
            for _, right:= range rightList{
                root := &TreeNode{Val : i}
                root.Left = left
                root.Right = right
                list = append(list, root)
            }
        }
    }
    
    return list
}