LeetCode 95: Unique Binary Search Trees II

Difficulty: 4

Frequency: 1

Problem:

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

Solution:

/** * 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) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<TreeNode *> answer;        if (n<=0)        {            answer.push_back(NULL);            return answer;        }        generateTreesRe(1, n, answer);        return answer;    }    void generateTreesRe(int i_begin, int i_end, vector<TreeNode *> & root)    {        if (i_begin==i_end)        {            root.push_back(new TreeNode(i_begin));            return;        }        if (i_begin>i_end)        {            root.push_back(NULL);            return;        }        vector<TreeNode *> left;        vector<TreeNode *> right;        for (int i = i_begin; i<=i_end; i++)        {            left.clear();            right.clear();            generateTreesRe(i_begin, i-1, left);            generateTreesRe(i+1, i_end, right);            for (int i_left = 0; i_left<left.size(); i_left++)            {                for (int i_right = 0; i_right<right.size(); i_right++)                {                    TreeNode * temp = new TreeNode(i);                    temp->left = left[i_left];                    temp->right = right[i_right];                    root.push_back(temp);                }            }        }    }};

Notes:

Some one use DP and DFS. I will consult others' code.