Create post order,IN order, Pre order, DFS,BFS examples in Kotlin

DFS/BFS pre/post order examples in Kotlin

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+Node data structure
+
+data class Node<T>(val value: T,
+                   var leftNode: Node<T>?,
+                   var rightNode: Node<T>?,
+                   var depth: Int = 0) {
+    fun link(left: Node<T>?, right: Node<T>?) = this.apply {
+        linkLeft(left).linkRight(right)
+    }
+
+    fun linkLeft(left: Node<T>?) = this.apply { leftNode = left }
+
+    fun linkRight(right: Node<T>?) = this.apply { rightNode = right }
+
+    fun depth(value: Int) = this.apply { depth = value }
+
+    /**
+     * Nodes on the left are in yellow, and those on the right are blue.
+     */
+    override fun toString(): String {
+        return StringBuffer().apply {
+            append("{${value.toString().green()}")
+            if (leftNode != null)
+                append(", ${leftNode.toString().yellow()}")
+            if (rightNode != null)
+                append(", ${rightNode.toString().blue()}}")
+        }.toString()
+    }
+}
+
+Building the tree
+
+/**
+ * [Image of the generated tree
+ *        [A]
+ *       /   \
+ *     [B]    [C]
+ *     / \    /  \
+ *  [D]  [E] [F] [G]
+ *               / \
+ *             [H] [I]
+ */
+fun buildTree(): Node<Char> {
+    val a = Node('a', null, null)
+    val b = Node('b', null, null)
+    val c = Node('c', null, null)
+    val d = Node('d', null, null)
+    val e = Node('e', null, null)
+    val f = Node('f', null, null)
+    val g = Node('g', null, null)
+    val h = Node('h', null, null)
+    val i = Node('i', null, null)
+
+    a.link(b, c)
+    b.link(d, e)
+    c.link(f, g)
+    g.link(h, i)
+
+    return a
+}
+
+Pre-order recursive traversal
+fun <T> traversalPreOrder(node: Node<T>?, list: MutableList<T>) {
+    if (node != null) {
+        list.add(node.value)
+        traversalPreOrder(node.leftNode, list)
+        traversalPreOrder(node.rightNode, list)
+    }
+}
+in-order traversals
+fun <T> traversalInOrder(node: Node<T>?, list: MutableList<T>) {
+    if (node != null) {
+        traversalInOrder(node.leftNode, list)
+        list.add(node.value)
+        traversalInOrder(node.rightNode, list)
+    }
+}
+post-order traversals
+fun <T> traversalPostOrder(node: Node<T>?, list: MutableList<T>) {
+    if (node != null) {
+        traversalPostOrder(node.leftNode, list)
+        traversalPostOrder(node.rightNode, list)
+        list.add(node.value)
+    }
+}
+BFS (breadth first search) using a Queue
+fun <T> breadthFirstTraversal(root: Node<T>): MutableList<Node<T>> {
+    val queue = LinkedList<Node<T>>()
+    val traversalList = mutableListOf<Node<T>>()
+
+    // Add first node.
+    queue.add(root)
+
+    // Use queue to create breadth first traversal.
+    while (queue.isNotEmpty()) {
+        val currentNode = queue.poll()
+        val depth = currentNode.depth
+
+        // Add left node first.
+        if (currentNode.leftNode != null)
+            queue.add(currentNode.leftNode!!.depth(depth + 1))
+
+        // Add right node next.
+        if (currentNode.rightNode != null)
+            queue.add(currentNode.rightNode!!.depth(depth + 1))
+
+        // Add the node to the traversal list.
+        traversalList.add(currentNode)
+    }
+
+    return traversalList
+}
+DFS (depth first search) using a Stack
+fun <T> depthFirstTraversal(root: Node<T>): MutableList<Node<T>> {
+    val stack = LinkedList<Node<T>>()
+    val traversalList = mutableListOf<Node<T>>()
+
+    // Add first node.
+    stack.push(root)
+
+    // Use stack to create breadth first traversal.
+    while (stack.isNotEmpty()) {
+        val currentNode = stack.pop()
+        val depth = currentNode.depth
+
+        // Push right child to stack FIRST (so this will be processed LAST).
+        if (currentNode.rightNode != null)
+            stack.push(currentNode.rightNode!!.depth(depth + 1))
+
+        // Push left child to stack LAST (so this will be processed FIRST).
+        if (currentNode.leftNode != null)
+            stack.push(currentNode.leftNode!!.depth(depth + 1))
+
+        // Add to traversal list.
+        traversalList.add(currentNode)
+    }
+
+    return traversalList
+}