給出n，生成所有由1…n為節點組成的不同的二叉查詢樹
樣例
給出n = 3，生成所有5種不同形態的二叉查詢樹：

#ifndef C164_H
#define C164_H
#include<iostream>
#include<vector>
using namespace std;
class TreeNode{
public:
    int val;
    TreeNode *left, *right;
    TreeNode(int val)
    {
        this->val = val;
        this->left = this
 
->right = NULL;
    }
};
class Solution {
public:
    /**
    * @paramn n: An integer
    * @return: A list of root
    */
    vector<TreeNode *> generateTrees(int n) {
        // write your code here
        vector<TreeNode *> vTree;
        if (n < 0)
            return vTree;
        vTree = build(1
 
, n);
        return vTree;
    }
    //構建start到end所有不同的二叉樹
    vector<TreeNode*> build(int start, int end)
    {
        vector<TreeNode*> vTree;
        if (start > end)
        {
            vTree.push_back(NULL);
            return vTree;
        }
        //對於i,左子樹為start到i-1,右子樹為i+1到end
 

        //並且遍歷左右子樹的每一種情況
        for (int i = start; i <= end; ++i)
        {
            vector<TreeNode*> left = build(start, i - 1);
            vector<TreeNode*> right = build(i + 1, end);
            for (int j = 0; j < left.size(); ++j)
            {
                for (int k = 0; k < right.size(); ++k)
                {
                    TreeNode* node = new TreeNode(i);
                    node->left = left[j];
                    node->right = right[k];
                    vTree.push_back(node);
                }
            }
        }
        return vTree;
    }
};
#endif