Recover a Binary Search Tree if positions of two nodes are swapped.

Two elements of a binary search tree (BST) are swapped by mistake. Restore the BST structure without changing positions of nodes which are correctly placed.

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The main idea that we are going to use is that in-order traversal array for a BST would be a sorted array.
If this order is not maintained, then we know that the nodes are not correctly placed.
Please see the pseudo-code below to understand the algorithm.

1. Initialize firstStartPoint = null, lastEndPoint = null, previous_node = null
2. Visit all nodes of a tree in in-order fashion. Keep track of previously visited node.
3. At each node that is being visited,
    if value of previously visited node > current node
        if(firstStartPoint == null)
            firstStartPoint = previous_node
         lastEndPoint = current_node;
4. After all nodes are visited :swap firstStartPoint with lastEndPoint     

Please check out the video for an illustrative explanation.

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Algorithm Visualization

Code Snippet

package com.ideserve.nilesh.questions;

public class RecoverBST
   static class TreeNode
		TreeNode left;
		TreeNode right;
		int val;
		public TreeNode(int x)
			this.val = x;

	TreeNode firstStartPoint, lastEndPoint;
    TreeNode prevNode;

    public void findSegments(TreeNode root) 
        if (root == null) return;
        findSegments (root.left);
        if (prevNode != null) 
            if (prevNode.val   >  root.val) 
                if (firstStartPoint == null)
                	firstStartPoint = prevNode;
                lastEndPoint = root;
        prevNode = root;
        findSegments (root.right);   
   public void recoverTree(TreeNode root) 
       int x = firstStartPoint.val;
       firstStartPoint.val = lastEndPoint.val;
       lastEndPoint.val = x;

   public void printInOrder(TreeNode root)
	   if (root == null) return;
   public static void main(String[] args)
	   TreeNode root = new TreeNode(10);
	   TreeNode n1   = new TreeNode(15);
	   TreeNode n2   = new TreeNode(5);
	   TreeNode n3   = new TreeNode(4);
	   TreeNode n4   = new TreeNode(7);
	   TreeNode n5   = new TreeNode(14);
	   TreeNode n6   = new TreeNode(17);
	   root.left  = n1;
	   root.right = n2;
	   n1.left  = n3;
	   n1.right = n4;
	   n2.left  = n5;
	   n2.right = n6;
	   RecoverBST solution = new RecoverBST();
	   System.out.println("In-Order traversal of BST before recovery: ");

	   System.out.println("In-Order traversal of BST after recovery: ");

Order of the Algorithm

Time Complexity is O(n)
Space Complexity is O(1)


  • Sincere thanks from IDeserve community to Nilesh More for compiling current post.

    Nilesh More

    Ninja Programmer