Construct Binary Search Tree from Preorder Traversal LeetCode solution in C++

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/**
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 * Definition for a binary tree node.
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 * struct TreeNode {
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 *     int val;
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 *     TreeNode *left;
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 *     TreeNode *right;
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 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
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 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
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 * right(right) {}
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 * };
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 */
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class Solution {
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 public:
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  TreeNode *help(int &i, vector<int> &preorder, int bound) {
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    if (i == preorder.size() || preorder[i] > bound) return NULL;
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    TreeNode *root = new TreeNode(preorder[i++]);
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    root->left = help(i, preorder, root->val);
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    root->right = help(i, preorder, bound);
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    return root;
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  }
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  TreeNode *bstFromPreorder(vector<int> &preorder) {
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    int i = 0;
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    return help(i, preorder, INT_MAX);
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  }
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};