Unique Binary Search Trees II in C++

Suppose we have an integer n, we have to generate all structurally unique binary search trees that store values from 1 to n. So if the input is 3, then the trees will be −
To solve this, we will follow these steps −
Define one recursive function called generate(), this will take low and high
define one tree node called temp.
if low > high, then insert null into the temp, and return temp
for i in range low to highleft_subtree := generate(low, i – 1)right_subtree := generate(i + 1, high)current := ifor j in range 0 to size of the left_subtreefor k in range 0 to size of the right_subtreecurr_node := make one tree node with value currentleft of the curr_node := left_subtree[j]right of the curr_node := right_subtree[k]insert curr_node into the temp
left_subtree := generate(low, i – 1)
right_subtree := generate(i + 1, high)
current := i
for j in range 0 to size of the left_subtreefor k in range 0 to size of the right_subtreecurr_node := make one tree node with value currentleft of the curr_node := left_subtree[j]right of the curr_node := right_subtree[k]insert curr_node into the temp
for k in range 0 to size of the right_subtreecurr_node := make one tree node with value currentleft of the curr_node := left_subtree[j]right of the curr_node := right_subtree[k]insert curr_node into the temp
curr_node := make one tree node with value current
left of the curr_node := left_subtree[j]
right of the curr_node := right_subtree[k]
insert curr_node into the temp
return temp.
Initially call the generate() function with values 1 and n to generate all trees.
Let us see the following implementation to get a better understanding −
 Live Demo
#include <bits/stdc++.h>
using namespace std;
class TreeNode{
   public:
      int val;
      TreeNode *left, *right;
      TreeNode(int data){
         val = data;
         left = right = NULL;
      }
};
void insert(TreeNode **root, int val){
   queue<TreeNode*> q;
   q.push(*root);
   while(q.size()){
      TreeNode *temp = q.front();
      q.pop();
      if(!temp->left){
         if(val != NULL)
            temp->left = new TreeNode(val);
         else
            temp->left = new TreeNode(0);
         return;
      }
      else{
         q.push(temp->left);
      }
      if(!temp->right){
         if(val != NULL)
            temp->right = new TreeNode(val);
         else
            temp->right = new TreeNode(0);
         return;
      }
      else{
         q.push(temp->right);
      }
   }
}
TreeNode *make_tree(vector<int> v){
   TreeNode *root = new TreeNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      insert(&root, v[i]);
   }
   return root;
}
void tree_level_trav(TreeNode*root){
   if (root == NULL) return;
      cout << "[";
   queue<TreeNode *> q;
   TreeNode *curr;
   q.push(root);
   q.push(NULL);
   while (q.size() > 1) {
      curr = q.front();
      q.pop();
      if (curr == NULL){
         q.push(NULL);
      }
      else {
         if(curr->left)
            q.push(curr->left);
         if(curr->right)
            q.push(curr->right);
         if(curr == NULL || curr->val == 0){
            cout << "null" << ", ";
         }
         else{
            cout << curr->val << ", ";
         }
      }
   }
   cout << "]"<<endl;
}
class Solution {
public:
   vector<TreeNode*> generate(int low, int high) {
      vector <TreeNode*> temp;
      if(low > high){
         temp.push_back(NULL);
         return temp;
      }
      for(int i = low;i<=high;i++){
         vector <TreeNode*> leftSubtree = generate(low,i-1);
         vector <TreeNode*> rightSubtree = generate(i+1,high);
         int current = i;
         for(int j = 0;j<leftSubtree.size();j++){
            for(int k =0;k<rightSubtree.size();k++){
               TreeNode* currentNode = new TreeNode(current);
               currentNode->left = leftSubtree[j];
               currentNode->right = rightSubtree[k];
               temp.push_back(currentNode);
            }
         }
      }
      return temp;
   }
   vector<TreeNode*> generateTrees(int n) {
      if(!n){
         vector <TreeNode*> r;
         return r;
      }
      return generate(1,n) ;
   }
};
main(){
   Solution ob;
   vector<TreeNode*> v = ob.generateTrees(3);
   for(int i = 0; i<v.size(); i++)
      tree_level_trav(v[i]);
}
3
[1, 2, 3, ]
[1, 3, 2, ]
[2, 1, 3, ]
[3, 1, 2, ]
[3, 2, 1, ]