二叉树结构：

public static class TreeNode{public int val;public TreeNode left;public TreeNode right;public TreeNode(int val) {this.val = val;}}

先序递归遍历：

public static void preOrder(TreeNode root){if (root == null){return;}System.out.print(root.val + " ");preOrder(root.left);preOrder(root.right);}

中序递归遍历：

public static void inOrder(TreeNode root){if (root == null){return;}inOrder(root.left);System.out.print(root.val + " ");inOrder(root.right);}

后序递归遍历：

public static void posOrder(TreeNode root){if (root == null){return;}posOrder(root.left);posOrder(root.right);System.out.print(root.val + " ");}

用栈改造先序递归遍历：

//用栈改造先序递归方法public static void preOrderUnRecur(TreeNode root){if (root == null){return;}Stack<TreeNode> stack = new Stack<>();stack.push(root);while (!stack.isEmpty()){TreeNode node = stack.pop();System.out.print(node.val + " ");//栈是先进后出  想要先打印左孩子  就要保证左孩子最后入栈if (node.right != null){stack.push(node.right);}if (node.left != null){stack.push(node.left);}}}

用栈改造中序递归遍历：

//用栈改造中序递归方法public static void inOrderUnRecur(TreeNode root){if (root == null){return;}Stack<TreeNode> stack = new Stack<>();while (!stack.isEmpty() || root != null){if (root != null){stack.push(root);root = root.left;}else {root = stack.pop();System.out.print(root.val + " ");root = root.right;}}}

用栈改造后序递归遍历：

//用栈改造后序递归方法public static void posOrderUnRecur(TreeNode root){if (root == null){return;}Stack<TreeNode> stack1 = new Stack<>();Stack<TreeNode> stack2 = new Stack<>();stack1.push(root);while (!stack1.isEmpty()){TreeNode node = stack1.pop();stack2.push(node);if (node.left != null){stack1.push(node.left);}if (node.right != null){stack1.push(node.right);}}while (!stack2.isEmpty()){TreeNode node = stack2.pop();System.out.print(node.val + " ");}}

提供一颗二叉树：

public static void main(String[] args) {TreeNode head = new TreeNode(1);head.left = new TreeNode(2);head.right = new TreeNode(3);head.left.left = new TreeNode(4);head.left.right = new TreeNode(5);head.right.left = new TreeNode(6);head.right.right = new TreeNode(7);preOrder(head);System.out.println();System.out.println("先序遍历递归版");inOrder(head);System.out.println();System.out.println("中序遍历递归版");posOrder(head);System.out.println();System.out.println("后序遍历递归版");preOrderUnRecur(head);System.out.println();System.out.println("先序遍历栈版");posOrderUnRecur(head);System.out.println();System.out.println("后序遍历栈版");inOrderUnRecur(head);System.out.println();System.out.println("中序遍历栈版");}