学步园

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);
    }
};