226. 翻转二叉树

示例 1：

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]
示例 2：

输入：root = [2,1,3]
输出：[2,3,1]
示例 3：

输入：root = []
输出：[]

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递归

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;

        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertTree(root.left);
        invertTree(root.right);
        return root;
    }
}

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层序遍历，然后每个节点的左右子树进行交换

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            // 交换左右子树
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;

            // 左右子树不空，加入队列
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
        return root;
    }
}