Solution

• 迭代法，中序遍历

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
// 中序遍历非递归  / 层序遍历也可以
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pRoot TreeNode类 
     * @return TreeNode类
     */
    TreeNode* Mirror(TreeNode* pRoot) {
        // write code here
        if (pRoot == nullptr) return nullptr;
        stack<TreeNode *> stack;
        TreeNode *node = pRoot;
        while (node || !stack.empty()) {
            while (node) {
                stack.push(node);
                node = node->left;
            }
            node = stack.top();
            stack.pop();
            swap(node->left, node->right);
            node = node->left;  // 翻转前的右子树
        }
        return pRoot;
    }
};

• 递归法，前/后序遍历

class Solution {
public:
    TreeNode* Mirror(TreeNode* pRoot) {
        // write code here
        if (pRoot == nullptr) return nullptr;
        Mirror(pRoot->left);
        Mirror(pRoot->right);
        swap(pRoot->left, pRoot->right);
        return pRoot;
    }
};