unique_trees_ii_dp.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{}
    }
    
    trees := make([][][]*TreeNode, n + 2)
    
    for i, _ := range trees {
        trees[i] = make([][]*TreeNode, n + 2)
    }
    
    return generateTreesInRange(1, n, trees)
}

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