lintcode:Unique Binary Search Trees II

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

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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

Tags

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    
private:

    TreeNode* clone(TreeNode *root) {
        
        if (root == NULL)
            return NULL;
            
        TreeNode *head = new TreeNode(root->val);
        head->left = clone(root->left);
        head->right = clone(root->right);
        
        return head;
    }

    vector<TreeNode *> generateTreesHelper(int start, int end) {
        
        vector<TreeNode *> retVtr;
        
        if (start > end)
        {
            retVtr.push_back(NULL);
            return retVtr;
        }
        
        TreeNode *root = NULL;
        for (int i=start; i<=end; i++) {
            
            root = new TreeNode(i);
            vector<TreeNode* > ltree = generateTreesHelper(start, i-1);
            vector<TreeNode* > rtree = generateTreesHelper(i+1, end);
            
            //雙重循環來構建左右子樹
            for (int l=0; l<ltree.size(); l++)
            {
                for (int r=0; r<rtree.size(); r++)
                {
                    root->left = ltree[l];
                    root->right = rtree[r];
                    retVtr.push_back(clone(root));
                }
            }
        }
        return retVtr;
        
    }
    
public:
    /**
     * @paramn n: An integer
     * @return: A list of root
     */
    vector<TreeNode *> generateTrees(int n) {
        // write your code here
        return generateTreesHelper(1, n);
    }
};