LeetCode之Unique Binary Search Trees II

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
/*采用类似Unique Binary Search Trees的思路，用递归的方法
求解出所有二叉搜索树。
参考自：https://github.com/soulmachine/leetcode*/
class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        return generateTrees(1, n);
    }
    
    vector<TreeNode*> generateTrees(int min_num, int max_num){
        vector<TreeNode*> trees;
        if(min_num > max_num){
            trees.push_back(nullptr);
            return trees;
        }
        for(int k = min_num; k <= max_num; ++k){
            vector<TreeNode*> left = generateTrees(min_num, k-1);
            vector<TreeNode*> right = generateTrees(k+1, max_num);
            for(int i = 0; i < left.size(); ++i){
                for(int j = 0; j < right.size(); ++j){
                    TreeNode* node = new TreeNode(k);
                    node->left = left[i];
                    node->right = right[j];
                    trees.push_back(node);
                }
            }
        }
        return trees;
    }
};