[leetcode] 95. Unique Binary Search Trees II

Given an integer 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

解法一：

根據數字i將n個數字分成左右兩邊，然後recursive的生成subtree。

/**
 * Definition for a binary tree node.
 * 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) {
        if(n==0) return {};
        return *generateTrees(1,n);
    }
    
    vector<TreeNode*>* generateTrees(int start, int end){
        vector<TreeNode*> *subtree = new vector<TreeNode*>();
        if(start>end) subtree->push_back(NULL);
        
        for(int i = start; i<=end; i++){
            vector<TreeNode*> *subleft = generateTrees(start,i-1);
            vector<TreeNode*> *subright = generateTrees(i+1,end);
            for(int j=0; j<subleft->size(); j++){
                for(int k=0; k<subright->size();k++){
                    TreeNode* cur = new TreeNode(i);
                    cur->left = (*subleft)[j];
                    cur->right = (*subright)[k];
                    subtree->push_back(cur);
                }
            }
        }
        
        return subtree;
    }
};