DSA/algorithms/Java/trees/tree-random-node.java

/** Author : Suraj Kumar
 * Github : https://github.com/skmodi649
 */

/** PROBLEM DESCRIPTION :
 * There is a Binary Search Tree given and we are supposed to find a random node in the given binary tree.
 */

/** ALGORITHM :
 * Step 1: START
 * Step 2: First create a binary tree using the steps mentioned in the first approach
 * Step 3: Now use a method inOrder() that takes a node as input parameter to traverse through the
 *         binary tree in inorder fashion as also store the values in a ArrayList simultaneously.
 * Step 4: Now define a method getrandom() that takes a node as input parameter, in this first call
 *         the inOrder() method to store the values in the arraylist, then find the size of the binary tree and now just generate a random number between 0 to n-1.
 * Step 5: After generating the number display the value of the ArrayList at the generated index
 * Step 6: STOP
 */


import java.util.ArrayList;

// Using auxiliary array to find the random node in a given binary tree
class Node {
    int item;
    Node left, right;

    public Node(int key) {
        item = key;
        left = right = null;
    }
}

class Tree {

    // Using a arraylist to store the inorder traversal of the given binary tree
    static ArrayList<Integer> list = new ArrayList<Integer>();
    // root of Tree
    Node root;

    Tree() {
        root = null;
    }

    // Now lets find the inorder traversal of the given binary tree
    static void inOrder(Node node) {
        if (node == null)
            return;

        // traverse the left child
        inOrder(node.left);

        list.add(node.item);
        // traverse the right child
        inOrder(node.right);
    }

    public void getrandom(Node val)
    {
        inOrder(val);
        // getting the count of node of the binary tree
        int n = list.size();
        int min = 0;
        int max = n - 1;
        //Generate random int value from 0 to n-1
        int b = (int)(Math.random()*(max-min+1)+min);
        // displaying the value at the generated index
        int random = list.get(b);
        System.out.println("Random Node : " + random);

    }


    public static void main(String[] args) {
        Tree tree = new Tree();


        tree.root = new Node(1);
        tree.root.left = new Node(12);
        tree.root.right = new Node(9);
        tree.root.left.left = new Node(5);
        tree.root.left.right = new Node(6);

        tree.getrandom(tree.root);


    }
}


/** Explanation of the Approach :
 * (a) Form the required binary tree
 * (b) Now use the inOrder() method to get the nodes in inOrder fashion and also store them in the given arraylist 'list'
 * (c) Using the getRandom() method generate a random number between 0 to n-1, then get the value at the generated random number
 *     from the arraylist using get() method and finally display the result.
 */


/** OUTPUT :
 * First output :
 * Random Node : 15
 * Second output :
 * Random Node : 99
 */

/** Time Complexity : O(n)
 * Auxiliary Space Complexity : O(1)
 */