leetcode——95——Unique Binary Search Trees II

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

/** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<TreeNode*> generateTrees(int n) {    if(n==0)           return vector<TreeNode*>{};       return generate(1, n);     }    vector<TreeNode*> generate(int begin,int end)    {        vector<TreeNode *> ret;        if(begin > end)            ret.push_back(NULL);        else if(begin == end)        {            TreeNode* node = new TreeNode(begin);            ret.push_back(node);        }        else        {            for(int i = begin; i <= end; i ++)            {//root                vector<TreeNode *> left = generate(begin, i-1);                vector<TreeNode *> right = generate(i+1, end);                for(int l = 0; l < left.size(); l ++)                {                    for(int r = 0; r < right.size(); r ++)                    {                        //new tree                        TreeNode* root = new TreeNode(i);                        root->left = left[l];                        root->right = right[r];                        ret.push_back(root);                    }                }            }        }        return ret;    }};