2 unique Binary Search Trees ii_leetcode

GivenN, Generate all structurally uniqueBST's(Binary Search Trees) that store values 1...N.

For example,
GivenN= 3, your program shocould return all 5 unique BST's shown below.

1 3 3 2 1

\///\\

3 2 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;    }};

 

2 unique Binary Search Trees ii_leetcode