递归

class Solution {
public:
    TreeNode* Mirror(TreeNode* pRoot) {
        //空树返回
        if(pRoot == NULL) 
            return NULL;
        //先递归子树
        TreeNode* left = Mirror(pRoot->left);  
        TreeNode* right = Mirror(pRoot->right);
        //交换
        pRoot->left = right; 
        pRoot->right = left;
        return pRoot;
    }
};

辅助队列层次遍历

class Solution {
public:
    TreeNode* Mirror(TreeNode* pRoot) {
        if(pRoot == nullptr) return nullptr;
        
        auto *newT = new TreeNode(pRoot->val);
        std::queue<TreeNode*> q;
        q.push(pRoot);
         
        while(!q.empty()){
            TreeNode* cur = q.front();
            q.pop();
            if(cur != nullptr){
                q.push(cur->left);
                q.push(cur->right);
                // 节点交换
                TreeNode*  tmp = cur->left;
                cur->left = cur->right;
                cur->right = tmp;
            }
        }
        return pRoot;

    }
};