2 Unique Binary Search Trees II_Leetcode

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

This is the second time I solve this problem.

Everytime when we encounter a BST problem without quite clear thought, we can resort to Divide & Conquer.

Use recursion to build the left and right subtree, then combine them then return.

In this problem, the left and right subtree can be multiple.

FIRST TRY ERROR: Forget to clear the vector of left of right subtree while apply different root.

Code:

/**
 * 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 *> generateTrees(int n) {
        vector<TreeNode *> res;
        if(n == 0)
        {
            res.push_back(NULL);
            return res;
        }

        res = gen(1, n);
        return res;
    }

    vector<TreeNode *> gen(int start, int end)
    {
        vector<TreeNode*> res;
        if(start == end)
        {
            TreeNode* tmp = new TreeNode(start);
            res.push_back(tmp);
            return res;
        }

        vector<TreeNode*> leftsub, rightsub;
        for(int i = start; i <= end; i++)
        {
            leftsub.clear();  // First try error
            rightsub.clear();  // First try error

            if(i == start) leftsub.push_back(NULL);
            else leftsub = gen(start, i-1);

            if(i == end) rightsub.push_back(NULL);
            else rightsub = gen(i+1, end);

            for(int m = 0; m < leftsub.size(); m++)
            {
                for(int n = 0; n < rightsub.size(); n++)
                {
                    TreeNode* root = new TreeNode(i); // divide & conquer
                    root->left = leftsub[m];
                    root->right = rightsub[n];
                    res.push_back(root);
                }
            }
        }

        return res;
    }
};