Recover Binary Search Tree

Problem

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?

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Idea

把BST树转入数组，然后从前往后扫数组，找到第一个顺序不对的数，然后从后往前扫，找到第二个顺序不对的。交换两个的val就ok了.

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Solution

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