题目：

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

For example,

Given n = 3, your program should return all 5 unique BST's shown below.

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

思路：

找到一个数作为根结点，剩余的数分别划入左子树或者右子树。需设置一个变量来记录左右子树能够生成的所有数。

/** * Definition for a binary tree node. * function TreeNode(val) { *     this.val = val; *     this.left = this.right = null; * } *//** * @param {number} n * @return {TreeNode[]} */var generateTrees = function(n) {    if(n==0){        return [];    }    return createTrees(1,n);};function createTrees(start,end){    var results=[];    if(start>end){        results.push(null);        return results;    }        for(var i=start;i<=end;i++){        var left=createTrees(start,i-1);        var right=createTrees(i+1,end);        for(var j=0;j