Construct Binary Search Tree from Preorder Traversal.cpp

/**
 * 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:
    TreeNode* help(int &i,vector<int>& preorder, int bound ){
        if(i==preorder.size() || preorder[i] > bound)
            return NULL;
         TreeNode* root = new TreeNode(preorder[i++]);
         root->left = help(i,preorder,root->val);
         root->right = help(i,preorder,bound);
        return root;
    }
    TreeNode* bstFromPreorder(vector<int>& preorder) {
     
        int i=0;
        return help(i,preorder,INT_MAX);
    }
};