Algorithm: Construct all numbers of a binary search tree

    package main
    
    import (
    	"fmt"
    	"sort"
    )
    
    /**
    1.给出一个数组，算出其能构成的不同二叉搜索树结构的数量
    */
    
    type array []int
    
    func (a array) Len() int {
    	return len(a)
    }
    
    func (a array) Less(i, j int) bool {
    	return a[i] < a[j]
    }
    
    func (a array) Swap(i, j int) {
    	a[i], a[j] = a[j], a[i]
    }
    
    // 二叉树节点
    type node struct {
    	val         int
    	left, right *node
    }
    
    func genBST(a array, l, h int) []*node {
    	var trees []*node
    	// 边界条件1