• LeetCode链接：226. 翻转二叉树

解法1 - 递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {    
	public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;
        invertTree(root.left);
        invertTree(root.right);
        TreeNode tmpNode = root.left;
        root.left = root.right;
        root.right = tmpNode;
        return root;
    }
}

解法2 - 非递归（层序遍历）

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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();
            if (node.left != null) queue.offer(node.left);
            if (node.right != null) queue.offer(node.right);
            TreeNode tmpNode = node.left;
            node.left = node.right;
            node.right = tmpNode;
        }
        return root;
    }
}