[LeetCode] Problem 897 - Increasing Order Search Tree

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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

No.1

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

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3
4
5
6
7
       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]

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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.

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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private TreeNode prev;

public TreeNode increasingBST(TreeNode root) {
    TreeNode dummy = new TreeNode(-1);
    prev = dummy;
    helper(root);
    return dummy.right;
}

private void helper(TreeNode root) {
    if (root == null)
        return;

    helper(root.left);

    prev.right = root;
    root.left = null;
    prev = root;

    helper(root.right);
}