897. Increasing Order Search Tree.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode increasingBST(TreeNode root) {
        // corner cases
        if (root == null) {
            return null;
        }
        
        return inorder(root, null);
    }
    
    private TreeNode inorder(TreeNode root, TreeNode tail) {
        // corner cases 
        if (root == null) {
            return tail;
        }
        
        TreeNode head = inorder(root.left, root);
        root.left = null;
        root.right = inorder(root.right, tail);
        return head;
    }
}