1008.cpp

#include <bits/stdc++.h>
#include <vector>
using namespace std;

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* bstFromPreorder(vector<int>& preorder) {
        int i = 0;
        return build(preorder, i, INT_MAX);
    }

    TreeNode* build(vector<int>& preorder, int& i, int bound) {
        if (i == preorder.size() || preorder[i] > bound) return nullptr;

        TreeNode* root = new TreeNode(preorder[i++]);

        root->left = build(preorder, i, root->val);
        root->right = build(preorder, i, bound);

        return root;
    }
};