Leetcode 95. Different binary search tree II-solving ideas and implemented in C ++

Problem-solving ideas:

The idea of ​​recursion.

 

/**
 * 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) {
        vector<TreeNode*> res;
        if(n < 1) return res;
        res = creatTree(1, n);
        return res;
    }
    vector<TreeNode*> creatTree(int start, int end){
        vector<TreeNode*> res;
        if(start > end){
            res.push_back(NULL);
            return res;
        }
        if(start == end){
            TreeNode* node = new TreeNode(start);
            res.push_back(node);
            return res;
        }
        for(int i = start; i <= end; i++){
            vector<TreeNode*> left = creatTree(start, i-1);
            vector<TreeNode*> right = creatTree(i+1, end);
            for(int j = 0; j < left.size(); j++){
                for(int k = 0; k < right.size(); k++){
                    TreeNode* node = new TreeNode(i);
                    node->left = left[j];
                    node->right = right[k];
                    res.push_back(node);
                }
            }
        }
        return res;
    }
};