LeetCode 226. 翻转二叉树

题目链接：https://leetcode-cn.com/problems/invert-binary-tree/

先序遍历，Java 代码：

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

        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        invertTree(root.left);
        invertTree(root.right);

        return root;
    }
}

后序遍历，Java 代码：

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

        invertTree(root.left);
        invertTree(root.right);

        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        return root;
    }
}

中序遍历，Java 代码：

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

        invertTree(root.left);

        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        invertTree(root.left);
        
        return root;
    }
}

层次遍历，Java 代码：

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

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();

            TreeNode tmp = node.left;
            node.left = node.right;
            node.right = tmp;

            if (node.left != null) {
                queue.offer(node.left);
            }

            if (node.right != null) {
                queue.offer(node.right);
            }
        }

        return root;
    }
}