java treenode如何实现遍历

在Java中，可以使用递归或迭代方法来遍历二叉树（TreeNode）。下面是两种遍历方法的示例：

1. 递归方法：

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

public class BinaryTreeTraversal {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);

        System.out.println("前序遍历：");
        preorderTraversal(root);

        System.out.println("\n中序遍历：");
        inorderTraversal(root);

        System.out.println("\n后序遍历：");
        postorderTraversal(root);
    }

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

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

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

2. 迭代方法（使用栈）：

import java.util.Stack;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

public class BinaryTreeTraversal {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);

        System.out.println("前序遍历：");
        preorderTraversal(root);

        System.out.println("\n中序遍历：");
        inorderTraversal(root);

        System.out.println("\n后序遍历：");
        postorderTraversal(root);
    }

    public static void preorderTraversal(TreeNode node) {
        if (node == null) {
            return;
        }
        Stack stack = new Stack<>();
        stack.push(node);
        while (!stack.isEmpty()) {
            TreeNode currentNode = stack.pop();
            System.out.print(currentNode.val + " ");
            if (currentNode.right != null) {
                stack.push(currentNode.right);
            }
            if (currentNode.left != null) {
                stack.push(currentNode.left);
            }
        }
    }

    public static void inorderTraversal(TreeNode node) {
        if (node == null) {
            return;
        }
        Stack stack = new Stack<>();
        TreeNode currentNode = node;
        while (currentNode != null || !stack.isEmpty()) {
            while (currentNode != null) {
                stack.push(currentNode);
                currentNode = currentNode.left;
            }
            currentNode = stack.pop();
            System.out.print(currentNode.val + " ");
            currentNode = currentNode.right;
        }
    }

    public static void postorderTraversal(TreeNode node) {
        if (node == null) {
            return;
        }
        Stack stack1 = new Stack<>();
        Stack stack2 = new Stack<>();
        stack1.push(node);
        while (!stack1.isEmpty()) {
            TreeNode currentNode = stack1.pop();
            stack2.push(currentNode);
            if (currentNode.left != null) {
                stack1.push(currentNode.left);
            }
            if (currentNode.right != null) {
                stack1.push(currentNode.right);
            }
        }
        while (!stack2.isEmpty()) {
            System.out.print(stack2.pop().val + " ");
        }
    }
}

这两种方法分别实现了前序遍历、中序遍历和后序遍历。你可以根据需要选择合适的方法来遍历二叉树。