Random node in BT: done

Author: rampatra

src/main/java/com/rampatra/trees/RandomNodeInBT.java

@@ -0,0 +1,82 @@
+package com.rampatra.trees;
+
+import java.util.Random;
+
+/**
+ * You are implementing a binary tree class from scratch, which has a method getRandomNode() which returns a
+ * random node from the tree. All nodes should be equally likely to be chosen. Design and implement an algorithm
+ * for getRandomNode().
+ *
+ * @author rampatra
+ * @since 2019-04-03
+ */
+public class RandomNodeInBT {
+
+    private static class TreeNode {
+        int val;
+        TreeNode left;
+        TreeNode right;
+        int size; // num of nodes in left subtree + 1 + num of nodes in right subtree
+
+        TreeNode(int val, int size) {
+            this.val = val;
+            this.size = size;
+        }
+
+        TreeNode getRandomNode() {
+            int randomNum = new Random().nextInt(this.size); // generates a random num from 0 to size - 1 (both inclusive)
+
+            /*
+                the below makes all nodes equally likely because the probability is distributed
+                evenly (approximately) depending on the number of children
+            */
+            if (this.left != null && randomNum < this.left.size) {
+                return left.getRandomNode();
+            } else if (this.right != null && randomNum > this.right.size) {
+                return right.getRandomNode();
+            } else {
+                return this;
+            }
+        }
+    }
+
+    public static void main(String[] args) {
+        /*
+            The BST looks like:
+            
+                      4
+                   /    \
+                  2      8
+                /  \   /  \
+               1   3  6   9
+             
+         */
+        TreeNode treeRoot = new TreeNode(4, 7);
+        treeRoot.left = new TreeNode(2, 3);
+        treeRoot.right = new TreeNode(8, 3);
+        treeRoot.left.left = new TreeNode(1, 1);
+        treeRoot.left.right = new TreeNode(3, 1);
+        treeRoot.right.left = new TreeNode(6, 1);
+        treeRoot.right.right = new TreeNode(9, 1);
+
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+        System.out.println(treeRoot.getRandomNode().val);
+
+        System.out.println("-------");
+
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+        System.out.println(new Random().nextInt(8));
+    }
+}