1008

/**
 * 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* bstFromPreorder(vector<int>& preorder) {
        int n = preorder.size();
        if(!n) return NULL;

        deque<TreeNode*> q;
        TreeNode *root = new TreeNode(preorder[0]);
        q.push_back(root);

        for(int i=1; i<n; i++){
            TreeNode *node = q.back();
            TreeNode *child = new TreeNode(preorder[i]);

            while(!q.empty() && q.back()->val < child->val){
                node = q.back();
                q.pop_back();
            }

            if(node->val < child->val)
                node->right = child;
            else
                node->left = child;

            q.push_back(child);
        }
        return root;
    }
};