- Given a binary tree, check whether given binary tree is binary search tree or not.
- Traverse the binary tree using depth first search (DFS) recursive algorithm.
- What is binary search tree (bst) ?
- Left child of binary tree is less than its parent node
- e.g. Node B is less than Node A (Fig 1).
- Right child of binary tree is greater than its parent node.
- e.g. Node C is greater than Node A (Fig 1).
- Binary Tree will be binary search tree, if above properties holds good for every node in a binary tree.
- Left child of binary tree is less than its parent node
Algorithm: check given binary tree is binary search tree in java
- Node A is root of binary tree.
- Node B is less than Node A
- Check left & right child of node B
- Node D is less than Node B
- Node E is greater than Node E
- Check left & right child of node B
- Node C is greater the Node A
- Similarly verify the properties with children of Node C.
- Above algorithm should suffice to check whether binary tree is BST.
Optimized algorithm to check given binary tree is binary search tree
- At every non leaf node, check the possible value ranges
- Node A can have value ranging from Integer.MIN_VALUE to Integer.Max_Value
- So, there is no restriction of value for root node of BST.
- What are the possible values of Node B?
- Node B is left child of Root A
- Node B can have any value that is less than A (i.e. <50)
- What are the possible values of Node C?
- Node C is right child of Root A
- Node C can have any value that is greater than A (i.e. >50)
- What will be possible value for Node F?
- As F is left child Node C and it can have value less than Node C (i.e. <60)
- Can we know what is minimum it can have?
- Yes, It can hold all values greater than Node A and less than Node C
- Node F > Node A & Node F < Node C
- What will be possible value range for Node E?
- Similar to Node F, Its range will be:Node E > Node B and Node E < Node A
Examples: check binary tree is binary search tree using recursive algorithm
Example 1: Check given binary tree is binary search tree
The above binary tree is binary search tree, as every node within its specified range.
Example 2: Given binary tree is not binary search tree.
- We have modified value Node H of Fig 2. We changed it to 90.
- We can see that Node H is less than Node F.
- It should be greater than Node A value (as it on right side A).
- Its violation of BST properties.
- Binary tree is not a BST.
Time Complexity of algorithm is O(n).
Program – Check given binary tree is binary search tree in java
1.) IsBST Class:
- IsBST class check whether given binary tree is binary search tree or not.
- Traverse the binary tree using recursive algorithm.
package org.learn.Question; import org.learn.PrepareTree.Node; public class IsBST { public static boolean isBST(Node node) { return IsBST.isBST(node, Integer.MIN_VALUE, Integer.MAX_VALUE); } private static boolean isBST(Node node, int min, int max) { if (node == null) return true; if (node.data < min || node.data > max) return false; boolean isLeft = isBST(node.left, min, node.data); if (!isLeft) return isLeft; boolean isRight = isBST(node.right, node.data, max); if (!isRight) return isRight; return true; } }
2.) Node Class:
- Node class representing the node of a binary tree.
package org.learn.PrepareTree; public class Node { public int data; public Node left; public Node right; public Node(int num) { this.data = num; this.left = null; this.right = null; } public Node() { this.left = null; this.right = null; } public static Node createNode(int number) { return new Node(number); } }
3.) App class:
- We are creating binary tree in a main method.
- We are calling method of IsBST class, to check given binary tree is binary search tree (or not).
package org.learn.Client; import org.learn.PrepareTree.Node; import org.learn.Question.IsBST; public class App { public static void main( String[] args ) { //root level 0 Node A = Node.createNode(100); //Level 1 Node B = Node.createNode(50); Node C = Node.createNode(150); //Level 2 Node D = Node.createNode(25); Node E = Node.createNode(75); Node F = Node.createNode(125); Node G = Node.createNode(175); //Level 3 Node H = Node.createNode(120); Node I = Node.createNode(140); Node J = Node.createNode(160); Node K = Node.createNode(190); //connect Level 0 and 1 A.left = B; A.right = C; //connect level 1 and level 2 B.left = D; B.right = E; C.left = F; C.right = G; //Connect level 2 and level 3 F.left = H; F.right = I; G.left = J; G.right = K; if(IsBST.isBST(A)) { System.out.println("Binary Tree is binary search tree");; } else { System.out.println("Binary Tree is not binary search tree"); } Node H1 = Node.createNode(90); F.left = H1; if(IsBST.isBST(A)) { System.out.println("Binary Tree is binary search tree");; } else { System.out.println("Binary Tree is not binary search tree"); } } }
Output – check given binary tree is BST in java (Fig 2 & Fig 3)
Binary Tree is binary search tree Binary Tree is not binary search tree