Traversal of binary trees, recursion and non-recursion

Import Java.util.arraydeque;import java.util.stack;class TreeNode {public int val;public TreeNode Left;public TreeNode Right;public TreeNode (int val) {this.val = val;}} public class binarytree{//first-order traversal recursive public static void preorder (TreeNode t) {if (t = = null) {return;} System.out.print (t.val+ "");p reorder (t.left);p reorder (t.right);} Middle order traversal recursive public static void Inorder (TreeNode t) {if (t = = null) {return;} Inorder (T.left); System.out.print (t.val+ ""); Inorder (t.right);} Post-traversal recursive public static void Postorder (TreeNode t) {if (t = = null) {return;} Postorder (T.left);p Ostorder (t.right); System.out.print (t.val+ "");} First-order traversal non-recursive public static void PreOrder2 (TreeNode t) {stack<treenode> s = new stack<> (); while (T!=null | |! S.isempty ()) {while (t! = null)//traverse the root node and the left subtree {System.out.print (t.val+ ""); S.push (t); t = T.left;} if (!s.isempty ())//Traverse right child {t = S.pop (); t = T.right;}}} Middle order traversal non-recursive public static void InOrder2 (TreeNode t) {stack<treenode> s = new stack<> (); while (T!=null | |! S.isempty ()) {while (t! = null) {S.push (t); t = T.left;} if (!s.isempty ()) {t = S.pop (); System.out.println (t.val); t = T.right;}}} Post-traversal non-recursive public static void PostOrder2 (TreeNode t) {stack<treenode> s = new stack<> (); stack<treenode> result = new stack<> ();//Store counter-sequential traversal result while (t! = null | |!s.isempty ()) {while (t! = null) {S.push (t ); Result.push (t); t = T.right;} if (!s.isempty ()) {t = S.pop (); t = T.left;}} while (!result.isempty ()) {System.out.print (Result.pop (). val+ "");}} Sequence traversal public static void Leveltraverse (TreeNode t) {arraydeque<treenode> queue = new arraydeque<> (); if (t = = NULL) {return;} Queue.offer (t); while (!queue.isempty ()) {t = Queue.poll (); System.out.print (T.val + ""); if (t.left! = null) {Queue.offer (t.left);} if (t.right! = null) {Queue.offer (t.right);}}} Spiral traverse public static void Spiraltraverse (TreeNode t) {stack<treenode> s1 = new stack<> (); stack<treenode> s2 = new stack<> (); if (t = = null) {return;} S1.push (t); while (!s1.isempty () | |!s2.isempty ()) {while (!s1.isempty ()) {t = S1.pop (); System.Out.print (t.val+ ""); if (t.right!=null) {S2.push (t.right);} if (t.left!=null) {S2.push (t.left);}} while (!s2.isempty ()) {t = S2.pop (); System.out.print (t.val+ ""); if (t.left!=null) {S1.push (t.left);} if (t.right!=null) {S1.push (t.right);}}}}

Traversal of binary trees, recursion and non-recursion