114. Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, given the following tree:

    1
   / \
  2   5
 / \   \
3   4   6

The flattened tree should look like:

1
 \
  2
   \
    3
     \
      4
       \
        5
         \
          6

Return tail of the flattened subtree

class Solution {
    public void flatten(TreeNode root) {
        go(root);
    }

    public TreeNode go(TreeNode root) {
        if(root == null) return null;
        if(root.left == null && root.right == null) {
            return root;
        }
        TreeNode tl = go(root.left);
        TreeNode tr = go(root.right);
        if(tr == null) {
            root.right = root.left;
            root.left = null;
            return tl;
        } else if(tl == null) {
            return tr;
        } else {
            tl.right = root.right;
            root.right = root.left;
            root.left = null;
            return tr;
        }
    }
}