LeetCode 99. Recover Binary Search Tree

99. Recover Binary Search Tree

Difficulty: Hard

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?

Solution

Language: Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode first, second, prev;
    public void recoverTree(TreeNode root) {
        if (root == null) {
            return;
        }
        prev = new TreeNode(Integer.MIN_VALUE);
        inorder(root);
        
        int tmp = first.val;
        first.val = second.val;
        second.val = tmp;
    }
    
    private void inorder(TreeNode root) {
        if (root == null) {
            return;
        }
        inorder(root.left);
        
        if (first == null && root.val <= prev.val) {
            first = prev;
        }
        
        if (first != null && root.val <= prev.val) {
            second = root;
        }
        
        prev = root;
        inorder(root.right);
    }
}