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99. Recover Binary Search Tree

{leetcode}/problems/recover-binary-search-tree/[LeetCode - Recover Binary Search Tree^]

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Example 1:

Input: [1,3,null,null,2]

   1
  /
 3
  \
   2

Output: [3,1,null,null,2]

   3
  /
 1
  \
   2

Example 2:

Input: [3,1,4,null,null,2]

  3
 / \
1   4
   /
  2

Output: [2,1,4,null,null,3]

  2
 / \
1   4
   /
  3

Follow up:

  • A solution using O(n) space is pretty straight forward.

  • Could you devise a constant space solution?

思路分析

二叉搜索树中序遍历时，就是一个升序排列的序列，在遍历过程中，就可以检查序列的大小。要求使用 O(1) 的空间复杂度，那么只能使用 Morris 遍历。

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一刷
link:{sourcedir}/_0099_RecoverBinarySearchTree.java[role=include]