Construct BST from Preorder

• Time : $O(n)$
• Space : $O(1)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
TreeNode* dfs(vector<int>& A, int& p, int mi, int ma) {
    if(p >= A.size()) return NULL;
    if(mi < A[p] and A[p] < ma) {
        TreeNode* res = new TreeNode(A[p++]);
        res->left = dfs(A,p,mi,res->val);
        res->right = dfs(A,p,res->val,ma);
        return res;
    }
    return NULL;
}
TreeNode* Solution::constructBST(vector<int> &A) {
    int p = 0;
    return dfs(A,p, INT_MIN, INT_MAX);
}