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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 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        return generateUtil(1, n);
    }
};