unique_trees_ii.go

// https://leetcode.com/problems/unique-binary-search-trees-ii/

/**
 * 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 generateTreesInRange(1, n)
}

func generateTreesInRange(min, max int) []*TreeNode {
    var trees []*TreeNode
    
    for val := min; val <= max; val++ {
        leftSubTrees := generateTreesInRange(min, val - 1)
        rightSubTrees := generateTreesInRange(val + 1, max)
        
        for _, left := range leftSubTrees {
            for _, right := range rightSubTrees {
                trees = append(trees, &TreeNode{
                    Val: val,
                    Left: left,
                    Right: right,
                })
            }
        }
    }
    
    if len(trees) == 0 {
        trees = append(trees, nil)
    }
    
    return trees
}