Unique Binary Search Tree 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.

 

Analyse: Reference at http://blog.csdn.net/sbitswc/article/details/25845567.

Runtime: 24ms.

/**
 * 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 generate(1, 0);
        return generate(1, n);
    }
    vector<TreeNode* > generate(int start, int end){
        vector<TreeNode* > subtree;
        if(start > end){
            subtree.push_back(nullptr); //what does it mean?
            return subtree;
        }
        for(int k = start; k <= end; k++){
            vector<TreeNode* > leftSubtree = generate(start, k - 1);
            vector<TreeNode* > rightSubtree = generate(k + 1, end);
            
            for(int i = 0; i < leftSubtree.size(); i++){
                for(int j = 0; j < rightSubtree.size(); j++){
                    TreeNode* root = new TreeNode(k);
                    root->left = leftSubtree[i];
                    root->right = rightSubtree[j];
                    subtree.push_back(root);
                }
            }
        }
        return subtree;
    }
};