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// Check preorder is valid or not
class Problem_19 {
// A binary tree node
static class Node {
int data;
Node left, right;
Node(int item) {
data = item;
left = right = null;
}
}
Node root;
// Function to insert a new node with given key in BST
static Node insert(Node node, int key) {
// If the tree is empty, return a new node
if (node == null) {
return new Node(key);
}
// Otherwise, recur down the tree
if (key < node.data) {
node.left = insert(node.left, key);
} else {
node.right = insert(node.right, key);
}
return node;
}
// Function to check if the given preorder traversal is valid for BST
static boolean isValidPreorder(int[] pre, int n) {
Node root = null;
// Insert elements of preorder array into BST
for (int i = 0; i < n; i++) {
root = insert(root, pre[i]);
}
// Perform inorder traversal of the constructed BST
int[] inorder = new int[n];
inorderTraversal(root, inorder, new int[]{0});
// Check if inorder traversal is sorted
for (int i = 1; i < n; i++) {
if (inorder[i] < inorder[i - 1]) {
return false;
}
}
return true;
}
// Helper function for inorder traversal
static void inorderTraversal(Node node, int[] inorder, int[] index) {
if (node == null) {
return;
}
inorderTraversal(node.left, inorder, index);
inorder[index[0]++] = node.data;
inorderTraversal(node.right, inorder, index);
}
// Driver method to test above methods
public static void main(String args[]) {
int[] pre1 = {10, 5, 1, 7, 40, 50};
int n1 = pre1.length;
if (isValidPreorder(pre1, n1)) {
System.out.println("Preorder 1 is valid for BST");
} else {
System.out.println("Preorder 1 is not valid for BST");
}
int[] pre2 = {10, 5, 40, 1};
int n2 = pre2.length;
if (isValidPreorder(pre2, n2)) {
System.out.println("Preorder 2 is valid for BST");
} else {
System.out.println("Preorder 2 is not valid for BST");
}
}
}