• 树中节点数目范围在 [0, 100] 内
• -100 <= Node.val <= 100

方法一：递归法

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        // 递归翻转左右子树
        TreeNode left = invertTree(root.left);
        TreeNode right = invertTree(root.right);
        
        // 交换左右子树
        root.left = right;
        root.right = left;
        
        return root;
    }
}

方法二：迭代法（使用栈） 

import java.util.Stack;

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) {
            return null;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            
            // 交换左右子树
            TreeNode temp = node.left;
            node.left = node.right;
            node.right = temp;
            
            // 将左右子树入栈
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        
        return root;
    }
}