LeetCode 897. Increasing Order Search Tree.java

Given a tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only 1 right child.

Example 1:
Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

       5
      / \
    3    6
   / \    \
  2   4    8
 /        / \ 
1        7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9  

Note:

1. The number of nodes in the given tree will be between 1 and 100.
2. Each node will have a unique integer value from 0 to 1000.

	/**
	* 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;
	}
	}

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