Inorder Traversal of Cartesian Tree

• Time : $O(n^2)$
• 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* helper(vector<int>& A, int l, int r) {
    if(l > r) return NULL;
    if(l == r) return new TreeNode(A[l]);
    int ma = max_element(begin(A) + l, begin(A) + r + 1) - begin(A);
    TreeNode* res = new TreeNode(A[ma]);
    res->left = helper(A,l,ma - 1);
    res->right = helper(A,ma + 1,r);
    return res;
}
TreeNode* Solution::buildTree(vector<int> &A) {
    return helper(A, 0, A.size() - 1);
}