java 写一个二叉树反转

二叉树反转，即将每个节点的左右子树交换位置。可以使用递归实现：

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

    TreeNode(int val) {
        this.val = val;
    }
}

public class BinaryTreeInversion {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        // 交换左右子树
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        // 递归处理左右子树
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}

在 main 方法中可以测试一下：

public static void main(String[] args) {
    /*
     *      4
     *    /   \
     *   2     7
     *  / \   / \
     * 1   3 6   9
     */
    TreeNode node1 = new TreeNode(1);
    TreeNode node2 = new TreeNode(2);
    TreeNode node3 = new TreeNode(3);
    TreeNode node4 = new TreeNode(4);
    TreeNode node6 = new TreeNode(6);
    TreeNode node7 = new TreeNode(7);
    TreeNode node9 = new TreeNode(9);

    node4.left = node2;
    node4.right = node7;
    node2.left = node1;
    node2.right = node3;
    node7.left = node6;
    node7.right = node9;

    BinaryTreeInversion inversion = new BinaryTreeInversion();
    TreeNode invertedTree = inversion.invertTree(node4);
    // 输出反转后的二叉树的前序遍历结果
    printPreOrder(invertedTree);
}

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

输出结果为：`4 7 9 6 2 3 1`，说明二叉树已经被反转了。