[LeetCode] C++: Intermediate Problem-Tree 95. Different Binary Search Trees II

95. Different Binary Search Trees II

Medium difficulty 776

Given an integer  n , generate all  binary search trees   composed of 1...  n nodes .

 

Example:

Input: 3
 Output: 
[ 
  [1,null,3,2], 
  [3,2,null,1], 
  [3,1,null,null,2], 
  [2,1,3], 
  [1,null ,2,null,3] 
] Explanation: The 
above output corresponds to the following five binary search trees with different structures: 
   1 3 3 2 1 
    \ / / / \ \ 
     3 2 1 1 3 2 
    / / \ 
   2 1 2 3

 

prompt:

• 0 <= n <= 8

Backtracking

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> generateTrees(int n) {
        if(!n){
            return {};
        }
        return generateTrees(1, n);
    }
    vector<TreeNode*> generateTrees(int start, int end){
        if(start > end){
            return {nullptr};
        }
        vector<TreeNode*> allTrees;
        for(int i = start; i <= end; i++){
            vector<TreeNode*> leftTrees = generateTrees(start, i-1);
            vector<TreeNode*> rightTrees = generateTrees(i+1, end);

            for(auto& left: leftTrees){
                for(auto& right: rightTrees){
                    TreeNode* curr = new TreeNode(i);
                    curr->left = left;
                    curr->right = right;
                    allTrees.emplace_back(curr);
                }
            }
        }
        return allTrees;
    }
};