Tree traversals 2

Author: AswinBarath

src/tree_traversals/Postorder_iterative_using_1_stack.java

@@ -1,9 +1,55 @@
 package tree_traversals;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Scanner;
+
+import tree_implementations.Node;
+
 public class Postorder_iterative_using_1_stack {
 
-	public static void main(String[] args) {
+	public static List<Integer> postorderTraversal(Node root) {
+		List<Integer> postorder = new ArrayList<Integer>();
 
+		return postorder;
+	}
+
+	static Scanner sc = null;
+
+	public static void main(String[] args) {
+
+		sc = new Scanner(System.in);
+
+		Node root = createTree();
+
+		List<Integer> ans = postorderTraversal(root);
+		System.out.print("Post order: ");
+		for (int data : ans) {
+			System.out.print(data + ", ");
+		}
+
+	}
+
+	static Node createTree() {
+
+		Node root = null;
+		System.out.println("Enter data: ");
+		int data = sc.nextInt();
+
+		if (data == -1)
+			return null;
+
+		root = new Node(data);
+
+		System.out.println("Enter left for " + data);
+		root.left = createTree();
+
+		System.out.println("Enter right for " + data);
+		root.right = createTree();
+
+		return root;
 	}
 
 }
+
+// Post order: 4, 5, 2, 8, 7, 6, 3, 1, 

src/tree_traversals/Postorder_iterative_using_2_stacks.java

@@ -1,9 +1,83 @@
 package tree_traversals;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Scanner;
+import java.util.Stack;
+
+import tree_implementations.Node;
+
+/*
+ * Time Complexity: O(N)
+ * 	where N is the no. of nodes in the tree
+ * Space Complexity: O(2N) (size due to usage of 2 stacks)
+ * 	where N is the no. of nodes in the tree
+ * (when queue stores every node - worst case)
+ * (don't take the space of answer DS into account)
+ */
 public class Postorder_iterative_using_2_stacks {
 
+	public static List<Integer> postorderTraversal(Node root) {
+		List<Integer> postorder = new ArrayList<Integer>();
+		Stack<Node> stack1 = new Stack<Node>();
+		Stack<Node> stack2 = new Stack<Node>();
+
+		if (root == null)
+			return postorder;
+
+		stack1.push(root);
+		while (!stack1.isEmpty()) {
+			root = stack1.pop();
+			stack2.push(root);
+			if (root.left != null)
+				stack1.push(root.left);
+			if (root.right != null)
+				stack1.push(root.right);
+		}
+
+		while (!stack2.isEmpty()) {
+			postorder.add(stack2.pop().data);
+		}
+
+		return postorder;
+	}
+
+	static Scanner sc = null;
+
 	public static void main(String[] args) {
-		
+
+		sc = new Scanner(System.in);
+
+		Node root = createTree();
+
+		List<Integer> ans = postorderTraversal(root);
+		System.out.print("Post order: ");
+		for (int data : ans) {
+			System.out.print(data + ", ");
+		}
+
+	}
+
+	static Node createTree() {
+
+		Node root = null;
+		System.out.println("Enter data: ");
+		int data = sc.nextInt();
+
+		if (data == -1)
+			return null;
+
+		root = new Node(data);
+
+		System.out.println("Enter left for " + data);
+		root.left = createTree();
+
+		System.out.println("Enter right for " + data);
+		root.right = createTree();
+
+		return root;
 	}
 
 }
+
+// Post order: 4, 5, 2, 8, 7, 6, 3, 1,