Added all binary tree traversal

Author: Prashant Jain

src/in/knowledgegate/dsa/binarytree/InorderTraversalIterative.java

@@ -8,7 +8,7 @@
 
 /**
  * Write Inorder traversal of a binary tree using
- * recursive approach
+ * iterative approach
  */
 public class InorderTraversalIterative {
   public List<Integer> inorderTraversal(TreeNode root) {

src/in/knowledgegate/dsa/binarytree/PostorderTraversalIterative.java

@@ -0,0 +1,42 @@
+package in.knowledgegate.dsa.binarytree;
+
+import in.knowledgegate.dsa.binarytree.model.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * Write Postorder traversal of a binary tree
+ * using iterative approach
+ */
+public class PostorderTraversalIterative {
+  public List<Integer> iterativePostorder(TreeNode root) {
+    List<Integer> result = new ArrayList<>();
+    if (root == null) {
+      return result;
+    }
+
+    Stack<TreeNode> s1 = new Stack<>();
+    Stack<TreeNode> s2 = new Stack<>();
+    s1.push(root);
+
+    while (!s1.isEmpty()) {
+      TreeNode temp = s1.pop();
+      s2.push(temp);
+
+      if (temp.left != null) {
+        s1.push(temp.left);
+      }
+      if (temp.right != null) {
+        s1.push(temp.right);
+      }
+    }
+
+    while (!s2.isEmpty()) {
+      result.add(s2.pop().val);
+    }
+
+    return result;
+  }
+}

src/in/knowledgegate/dsa/binarytree/PostorderTraversalRecursive.java

@@ -0,0 +1,26 @@
+package in.knowledgegate.dsa.binarytree;
+
+import in.knowledgegate.dsa.binarytree.model.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Write Postorder traversal of a binary tree
+ * using recursive approach
+ */
+public class PostorderTraversalRecursive {
+  public List<Integer> postorderTraversal(TreeNode root) {
+    List<Integer> list = new ArrayList<>();
+    postorderTraversal(root, list);
+    return list;
+  }
+
+  private void postorderTraversal(TreeNode root,
+      List<Integer> result) {
+    if (root == null) return;
+    postorderTraversal(root.left, result);
+    postorderTraversal(root.right, result);
+    result.add(root.val);
+  }
+}

src/in/knowledgegate/dsa/binarytree/PreorderTraversalIterative.java

@@ -0,0 +1,36 @@
+package in.knowledgegate.dsa.binarytree;
+
+import in.knowledgegate.dsa.binarytree.model.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Stack;
+
+/**
+ * Write Preorder traversal of a binary tree using
+ * iterative approach
+ */
+public class PreorderTraversalIterative {
+  public List<Integer> iterativePreorder(TreeNode root) {
+    List<Integer> result = new ArrayList<>();
+    if (root == null) {
+      return result;
+    }
+
+    Stack<TreeNode> stack = new Stack<>();
+    stack.push(root);
+    while (!stack.isEmpty()) {
+      TreeNode temp = stack.pop();
+      result.add(temp.val);
+
+      if (temp.right != null) {
+        stack.push(temp.right);
+      }
+      if (temp.left != null) {
+        stack.push(temp.left);
+      }
+    }
+
+    return result;
+  }
+}

src/in/knowledgegate/dsa/binarytree/PreorderTraversalRecursive.java

@@ -0,0 +1,26 @@
+package in.knowledgegate.dsa.binarytree;
+
+import in.knowledgegate.dsa.binarytree.model.TreeNode;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Write Preorder traversal of a binary tree using
+ * recursive approach
+ */
+public class PreorderTraversalRecursive {
+  public List<Integer> preorderTraversal(TreeNode root) {
+    List<Integer> result = new ArrayList<>();
+    preorderTraversal(root, result);
+    return result;
+  }
+
+  public void preorderTraversal(TreeNode root,
+                                List<Integer> result) {
+    if (root == null) return;
+    result.add(root.val);
+    preorderTraversal(root.left, result);
+    preorderTraversal(root.right, result);
+  }
+}