Two elements of a binary search tree (BST) are swapped by mistake. Recover the tree without changing its structure. Note: A solution using O(n) space is pretty straight forward. Could you devise a constant space solution? confused what"{1,#,2,3}"means? > read more on how binary tree is serialized on OJ. OJ's Binary Tree Serialization: The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below. Here's an example: 1 / \ 2 3 / 4 \ 5 The above binary tree is serialized as"{1,2,3,#,#,4,#,#,5}".