bfs

TreeNode* Mirror(TreeNode* pRoot) {
        // write code here
        if(!pRoot) return nullptr;
        queue<TreeNode*> q;
        q.push(pRoot);
        while(!q.empty())
        {
            int s = q.size();
            for(int i=0;i<s;i++)
            {
                TreeNode* l = q.front()->left;
                TreeNode* r = q.front()->right;
                if(l) q.push(l);
                if(r) q.push(r);
                q.front()->right = l;
                q.front()->left = r;
                q.pop();
            }
        }
        return pRoot;
    }