897.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution { // Runtime: 36 ms, faster than 94.74% of C++ online submissions
                 // for Increasing Order Search Tree.
public:
  TreeNode *increasingBST(TreeNode *root) {
    TreeNode *head = nullptr, *ret = nullptr;
    function<void(TreeNode *)> inorder = [&](auto node) {
      if (not node)
        return;
      inorder(node->left);
      if (not head)
        ret = head = node;
      else
        head = head->right = node;
      head->left = nullptr;
      inorder(node->right);
    };
    inorder(root);
    return ret;
  }
};