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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

生成不相同的二叉树

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