LeetCode: 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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

利用递归，从低向上依次建立BST。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    void build(vector<TreeNode*> &result, int nStart, int nEnd)    {        if (nStart > nEnd)            result.push_back(NULL);                else        {            for (int i = nStart; i <= nEnd; ++i)            {                vector<TreeNode*> left;                build(left, nStart, i-1);                vector<TreeNode*> right;                build(right, i+1, nEnd);                for (int j = 0; j < left.size(); ++j)                {                    for (int k = 0; k < right.size(); ++k)                    {                        TreeNode* root = new TreeNode(i);                        root->left = left[j];                        root->right = right[k];                        result.push_back(root);                    }                }            }        }    }        vector<TreeNode *> generateTrees(int n) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<TreeNode*> result;        build(result, 1, n);        return result;    }};